It reads to me as if you are valuing equally over the whole encounter. An enemy casting a control spell on round 4 isn’t nearly the same as casting that spell on round 1. If bless is up on round 1 it will make a much larger difference in that encounter than if the enemy gets 1 extra round in which he uses that same control spell on round 4 (IMO starting tier 2 bless is likely to cause more damage than an equal level guiding bolt *some exceptions apply)
You seem to be getting half my point, but missing the other half.
If you cast it in Rd 1, there is no immediate
direct change on the battlefield. Nobody falls down. Nobody loses hps. You're altering the attack probability and the saving throw probability of future situations, and in both cases you're doing so in a manner that will matter in 1 in 8 rolls. So, on average, you're going to get 1 extra hit out of 8 attack rolls, and you're going to make 1 extra saving throw once in every 8 saving throws. This is (as a 1st level spell) going to be limited to 3 creatures, so at low levels those 8 attack rolls will require about 3 rounds (or less if you dual wield, but then your damage is less per attack, so it offsets).
That extra hit has to match the average benefit of a guiding bolt for that extra hit to have been equal in damage benefit of a guiding bolt. That average benefit, when a GB hits, is ~14 damage plus the expected average damage increase for that advantage being granted to the next strike and offset by the impact of the chance you miss with the GB. If one hit comes up shy of equivalent damage, then you need to double the number of attacks to 16 before your bless is expected to account for more damage than the guiding bolt - which could be 5 to 6 rounds. By that time, if your combat is not in cleanup mode, you're in an unusual combat.
On the defensive side, bless benefits 1 in 8 saving throws. Often, there will not be enough saving throws made in a combat that the bless will have any real impact defensively. However, the defensive penalty of using bless instead of guiding bolt is more likely to impact the party.
By not directly doing damage on your turn, and instead giving 3 members of your team a buff, you slowed down how quickly you could take monsters down. That means the monsters may get an extra attack. And that extra attack may not be just one attack. If it takes you longer to fell the first monster, that means it will be longer before you start damaging the next monster. Casting bless instead of guiding bolt may result in several extra attacks during a combat being delivered to the party.
Example:
Rd 1 version A: Cleric casts bless, then ogre attacks, then ally fighter attacks, then ogre 2 attacks, then ally rogue attacks, then ogre 3 attacks, then ally paladin attacks. We see three attacks, so there has been 3 opportunities for there to have been a miss turned into a hit, which works out in a ~33% chance that the bless had an impact on the combat.
For comparison: Rd 1 version B: Cleric casts guiding bolt, then ogre attacks, then ally fighter attacks, then ogre 2 attacks, then ally rogue attacks, then ogre 3 attacks, then ally paladin attacks. Cleric opens up with a chance to deal ~14 damage, and increase the chance the fighter hits. At the end of the fighter turn, you're clearly better off and have a bigger head start on taking down a foes.
From there on, there is a 1 in 8 chance that the bless spell would have assisted you. There is a less than 25% chance bless would help you in those last two attacks during the first round. As we close out the first round, there is a good chance the version A party either dropped an ogre, or are about to do so. This means you're likely to see one less attack from that ogre than you otherwise would see if you cast bless. This extra attack outweighs the benefit of the save bonus in most combats, as you need about 8 saving throws amongst 3 PCs in combat to average 1 meaningful benefit.
As the rounds progress, bless - if it is not lost due to a loss of concentration - can catch up on the damage dealt, but it is unlikely to make up for defensive penalty of that extra round of attacks from ogres until after the offensive benefit overtakes the GB in 5 to 7 rounds. Further, you're also expending your concentration during that time, which means other concentration benefit spells are off the table.
There are changes to the situation that can increase or decrease the benefit of bless (such as enemies that force a lot of saves, casting it before combat, etc...), but in general, the benefit of it is extremely overestimated in this edition, and it gets used more than it should.
If you're still not convinced: You also have to factor in the tendency for PCs to use their best attacks first in a combat, meaning that unless the cleric wins initiative, the bless is also often going into effect after some of the most meaningful attack rolls of the combat are in place.
I have many PCs that have this spell. It is not cast often.