Tempest, by a large margin. Good AoEs, and maximized damage.
And while it's technically not a Cleric, but still Divine, the new UA Wildfire Druid makes a dandy fire-based blaster, with Firebolt, Create Bonfire, Produce Flame, Flaming Sphere, Scorching Ray, Heat Metal, Fireball, Flame Arrows, Wall of Fire, Flame Strike, etc. Add on the Elemental Adept: Fire feat for even more fun. (And you still have good alternate element spell options for the times you run into Fire Immunity.)
Just as a straight blaster I think Light might outdo Tempest (or at least look like it) but I don't like the mono damage focus on the most commonly resisted damage type, and Tempest has better control spells, which I care about more than blasting anyway. Tempest is the better choice IMO. Still a fine blaster and with better peripherals.
Call Lightning is the only spell affected, since it requires either an open outdoor area, or an indoor area that is at least 120 feet wide and over 15ish feet tall. (The original requirement of a 100 foot tall ceiling was errata'd out; now you just have to have room overhead for a 10 foot tall cloud.) So you'll occasionally come across large indoor rooms/caves where Call Lightning can still be used.
And you still have Thunderwave and Shatter (upcast and maximized, as needed) for AoE blasting in situations where Call Lightning isn't practical, plus Destructive Wave and Flame Strike, and Spirit Guardians for close-range AoE damage (the latter two are available to all Clerics).
Consider that a Thunderwave or Shatter spell, cast at 3rd level and Maximized, does 32 damage on a failed save. That's 4 points more than the average damage of a 3rd level Fireball from a Light Cleric. A 4th level upcast and maximized Thunderwave/Shatter does 8.5 points more than the average damage of a 4th level upcast Fireball. Granted, Fireball covers a larger area, but the smaller area of Thunderwave/Shatter is handy for careful positioning to avoid friendlies. And they're both doing Thunder damage, which is a less-resisted element than Fire.
Call Lightning does similar maximized damage to Thunderwave/Shatter (30 at 3rd, 40 at 4th), with the big difference being that instead of just doing that damage once, it can do its damage once per round for 10 minutes, all for the cost of one spell slot and your Concentration. (Okay, so no combat is going to last 100 rounds, but it's still very economical...) Only the first two bolts would be able to be Maximized, but then you're still tossing out an average of 16.5/21 damage with the up to 98 subsequent bolts. And since it's Lightning damage, you can push enemies around with those bolts, thanks to your Thunderbolt Strike ability.
Besides the ability to throw fireballs, the light domain cleric also has radiance of the dawn, a 30 foot AoE effect which does radiant damage (not fire) and only harms enemies. As a ribbon ability it also dispels darkness. This is very useful for taking out a large number of weak, low-level enemies. In general I find that fireball makes the light cleric good at long range bombs while spirit guardians and radiance of the dawn make it dangerous close up.
Playing a tempest cleric in a campaign now, and finding him to be quite an effective blaster, at least through low tier 2. Light cleric might be better at very low levels, simply because they can blast more often and in a party-friendly way, but their CD doesn't scale well.
I like both but for a cleric that focuses more on casting, I'd probably go with Light. Light's spell list is very versatile - mix of close and long range, single-target and AoEs. Radiance is a very good AoE and doesn't require spell slots to boot. Warding Flare adds some decent cc and they get one of the best buffs spells in the game: Faerie Fire.
Tempest still works well as a DEX-based, light armor wearing ranged caster/bow user. Especially with a little bit of multiclassing, like Fighter 1 for Archery Fighting Style or Ranger 2 for Archery Fighting Style, Absorb Elements, and Hunter's Mark.
Other than Faerie Fire, I don't find the Light domain spells to be that exciting. Burning Hands is a worse Thunderwave. Flaming Sphere is mostly a worse Spiritual Weapon (particularly due to its concentration requirement and full action initial casting time). Scorching Ray doesn't do that much more than spiritual weapon + weapon attack in the round you use it, and is even with spiritual weapon + toll the dead once you hit 5th, and then does nothing in subsequent rounds. Fireball is a good blast spell of course, but other than its bigger area (which is a mixed blessing), it does less than a tempest cleric upcasting and maximizing Shatter.
The light cleric's blaster status really lives and dies on Radiance of the Dawn, it seems to me.
At Tier 2, Level 2 spells start to matter less. You can be upcasting that Spiritual Weapon into a 4th level slot for double dice on each hit. Or upcasting that Maximized Thunderwave, Shatter, or Call Lightning into a 4th or 5th level slot for a flat 40 or 48/50 damage. While Radiance of the Dawn is still doing 2d10 + 7/8/9.
You are correct that Thunderwave/Shatter are CON saves, which are less ideal in some situations. But Call Lightning is a DEX save. And it does its damage round after round, with one casting. So while you may be saving the equivalent of 2nd level slots with Radiance of the Dawn, you're saving 3rd (or higher) slots with Call Lightning. And it uses your Action on subsequent turns to call down additional bolts, which leaves your Bonus Actions free, allowing it to still be used in conjunction with Spiritual Weapon, Healing Word, etc.
Radiance of the Dawn is a fine ability, for the reasons you've pointed out. But look at it this way, apples to apples: It's a less powerful Channel Divinity ability than maximizing your thunder/lightning damage, especially at higher levels (5+). And Fireball is a less powerful 3rd level domain spell than Call Lightning. Faerie Fire is a great spell, when your Concentration isn't in use elsewhere, but not enough to make up the difference. So the Tempest Cleric pulls well ahead as far as blasters go, in my book.
If you're open to semi-official, look at Zeal Domain from Plane Shift:Amonkhet. It gains the maximize ability of the Tempest domain, except instead of thunder and lightning, it's thunder and fire. Oh, and it gets fireball as a domain spell.