An attack is not the same as a hit. This varies based on AC, but an attack is only worth maybe .7 of a hit on average and a lot less than that on a very high AC enemy where Bless is at its best.
Now that I have a bit time on my hands, I try to do a proper calculation in your scenario, using the cleric and two champion fighters.
For example: 8th level Cleric fighting a 22AC enemy:
AC ranges at CR 8 are 14 - 18. Probably somwhere around 16 on average.
Cleric Swings his axe for 12 damage on a hit (1d8+1d8+3) with a +6 attack. His DPR is 3.
In that case it is an average of 6.6 damage on a hit. +6 does not sound that unreasonable, probably due to 15 str and a +1 weapon. Could also be 16 str amd a magic weapon if the cleric considers a weapon attack to be a good choice. So it might also be 0.6*13. But probably the better course of action would be casting either sacred flame (2d8+4 against dex), toll the dead(2d8 or even 2d12+4 against wounded targets against wisdom) or word of radiance (2d6+4 against multiple targets against constitution) instead, divine spark does 2d8+4 damage against consitution).
Lets just assume a single unhurt target and either wis or dex is an ok save to target. Lets just assume +2 to save on average on the worst save. DC is 15. So also 0.6*13. So 7.8 damage.
He loses 3 damage in round 1 casting bless.
Now they probably lose 7.8 damage but gained the possibility of casting a bonus action spell on top. Spiritual weapon or so. Don't count it in, but still note that as opportunity cost.
A level 1 spell could be sanctuary (no concentration) or shield of faith for some added protection for a front liner (we assume enemies use attack rolls, otherwise bless is obviously a good choice).
A spell that replaces the cantrip could be the not well liked inflict wounds if con is the worst saving throw (again, assume a bonus of +2 against DC 15).
It does 11*0.6+5.5*0.4 = 8.8 damage on average.
All that damage is front loaded.
After he casts bless his DPR goes up from 3 to 3.9 so, so in a 3 round fight he does 7.8 vs 9 damage so it cost a little more than a hit point of damage for him to cast bless.
You assume the cleric attacks every round. They might want to cast spells in the next round. In that case, the bonus is wasted on the cleric. And all the +1d4 on themself does is allowing to hold conventration better... for bless itself.
The fighters he cast it on get 4 attacks a round combined (if they don't action surge).
They should action surge as soon as possible.
At 8th level they are doing 12 damage too and are doing it with a +8.
18 Str and a +1 weapon seems fair. Or even 20 Str and a +1 weapon.
I assume you use a great sword. So 2d6+6 in my books. And since I consider GWF and GWM as good (enough) feats, I guess we should add +4 extra damage on average.
But I also assume graze.
So their DPR is 8.4 (two attacks).
So their real DPR is (0.7*17+0.3*5+0.1*7)
2+(1-0.9^2)(0.7*14+0.3*5+0.1*7) =~ 30.5 without using action surge. Chance for an extra attack on a crit factored in. Assumed champion subclass.
(0.7*17+0.3*5+0.1*7)*2 = 28.2 extra using action surge for a total of 58.7 (56.13 without champion subclass) damage against an average enemy.
2 of those fighters do around 117.5 damage on the first round of combat.
If both fighters are blessed, chances increase by 2.5/20=0.125 per hit.
With bless it goes up to 10.2. That is each, so overall it is 16.8 vs 20.4. per round.
So those two fighters deal
((0.825*17+0.175*5+0.1*7)
4+(1-0.9^2)(0.825*14+0.175*5+0.1*7)) *2 = 129.7875 on average in the first round of combat.
Against 125.3 if the cleric just attacks.
So in the very first round they have already made up everything the Cleric lost casting Bless.
Assuming the cleric goes first.
Over the course of 3 rounds the damage is 59.4 no bless vs 69 with bless.
So. I just used one round of combat. Let's see why:
135 is a common HP pool for CR 8 creatures. CR is a low to medium encounter for a 3 person party of level 8.
One where preserving slots makes sense.
This is also a very good level for fighter DPR. 3 feats to get str to 20. The cleric gained nothing noteworthy on that level (wis probably went up to 19).
So the damage done to the average low to medium challenging monster is high enough without using a spell slot at all to kill the beast on round 2, and using that spell slot will probably not allow you to kill the beast in round 1.
This also does not consider weapon masteries or damage buffs or anything else that would make the numbers even bigger and it doesn't consider the save bonus you get either.
I did consider the most likely weapon mastery. Topple is a bit more difficult to calculate. I will do a this claculation later.
This is a legit question. I don't think a Cleric Cantrip is generally going to do better in tier 2 unless the enemy is already damaged and has a very weak Wisdom save. Other leveled spells might work better, but other first level spells using a first level slot only situationally so.
Still correct.
But now backed us with more realistic numbers, we see that bless is only increasing damage by around 4 points on average. Considering that the party does around 125 damage, that is just a bit more than a 3% increase in total damage. Probably not worth the slot. And if the cleric does not go first (which you assumed), it is even less.
If the cleric has alert, they will go first, but this is another opportunity cost paid. Instead they could be a sage and have taken true strike with magic initiate or be a high elf instead of a human. (one feat against one cantrip).
True strike will increase the damage output of the cleric from 7.8 on average to 0.65*(2d8+1d6+5)=0.65*17.5=~11.4
That almost makes up the total difference between using bless or not.
Magic Missile is a pretty solid 1st level spell especially against a high AC, if the Cleric has it and casts it round 1 vs that 22AC baddie that is 66.9 vs 69.
Inflict wounds is a bit on the weak side compared to that. 3d4+3 averages 10.5.
But clerics usually don't get magic missile. 8.8 hower is not extraordinary terrible. Probably not worth the slot though. They should not have nerfed it. Making it a save instead of an attack roll would have been sufficient to lower the chance to instant kill a level 1 PC with a crit.
Witchbolt is a good 1st level spell. If he casts that on round 1 65.5 vs 69
Also no cleric spell.
Chromatic Orb is a solid spell generally, assuming 2 enemies is 61.7 vs 69.
Also no cleric spell.
Guiding Bolt which is the only one of these options most Clerics have is 62.2 vs 69
Guiding bolt against AC 16 might actually be a useful spell if the topple mastery is used by one of the fighters.
A 60% chance to grant advantage and deal 4d6 damage is not terrible at all.
Btw: the cleric did not chose a subclass.
A light cleric could use radiance of the dawn for 2d10+8, save half damage (vs constitution), probably dealing around 15 (area) damage on average, enough to mame up for the loss of bless.
Or cast burning hand for reliable damage (and preventing regeneration), or fairy fire for constant advantage if dex is the save is bad on the target.
A trickery cleric could use invoke duplicitely if they decided to attack in melee for advantage.
War cleric is a tricky one. +10 to hit will make sure one attack that would miss will now hit for certain. And they can cast spiritual weapon without concentration on top of casting bless.
So, I will stand by my assessment. Bless is a very good spell in certain situations. But it is not worth casting all the time, due to the fact that in the cases where it is good, it is probably not needed at all, and where it is great, using higher level spells have higher priority.
Great situations are, when you can precast it and a boost to saving throws is needed in the first round of combats.
