My blue dragon quick and dirty review
1) I like static charge. Immediately differentiates the previous dragons and feels well within flavor.
2) Multi-attack needs a cleanup. Looks like the substitution is written backwards, I think you mean substitute 1 bite or 2 claws in order to gain a 1 tail attack but it reads the opposite way. That said, I like the notion that blue dragons are better with their tails and so include that in their routine, a subtle but nice touch.
3) I like giving the blues a horn attack...but I would like to see the effect more differentiated, as there already a lot of proning in the core dragon routine. So what if it was more like a gore attack, or something that did wounding damage, or maybe you grappled on the horn or something. I think there is more space to play with that attack to make it interesting.
4) The thunderclap damage. This is where the perception differentiates from the paper. On paper the damage all adds up, perception the fact that a dragon is using an entire "action" and dealing 5 damage.... to characters capable of 7-9th level spells.... its like "why bother". All it does is trigger death saving throws (and the strip resistance of course), which is a solid purpose, but its feels out of flavor with a mighty and powerful dragon. This is different of course from the static charge or ignite abilities...because those are more "automatic", they just happen during the battle, and so can add up, as compared to a dragon's "action".
But here's an easy way to clean it up without changing the numbers....make the damage automatic. Alright you all are hit....take 5 damage, and now make a saving throw. Players are very funny about automatic damage, it immediately scares them, even if its low. I still remember the time I used a monster (reskinned from just a simple magic missile wielding mage). Even though I was doing about the damage a party would take even if they passed a fireball saving throw...they freaked out, because the damage was "unstoppable".
So by making the damage automatic it adds a little bit of terror in, even though realistically the damage is nothing at those levels. It also makes the ability easier for the dm. If the party doesn't have lightning resistance (either because they didn't have it or its already been stripped), the DM can just give out damage without doing a whole round of saves, speeds things up a bit.
5) Breath: My only note, i would increase the width to 10 ft. This is mainly because as long as it is, I always find lines are so much weaker than cones in the vast majority of scenarios. A 10 ft width gives you just a little bit of room to maneuver. I like that the breath seems stronger in damage, weaker in effect, and weaker on magic strip....like you said more of an artillery weapon.
If you wanted to differentiate it just a bit more, you could do something with recharge to show that the dragon is literally "powering up" and is now teeming with electricity. I mention this because as an artillery piece I imagine the "breath + recharge + breath" maybe more common than the "breath + multi + multi" routine.
Recharge: You regain the use of your breath. Until the start of your next turn, your static charge damage is doubled.
