I don't do Jud9s speedruns (anymore), so I will abstain from voting.
However, I do believe that changing health is a GOOD thing from a design perspective. Allow me to explain why, because I don't think anyone else in the thread has done a thorough enough job explaining it. To do so, I have to talk about Devil Deals in general.
In the Jud6s mod, DDs are changed to always give 1 item. This affects balance in a non-trivial way. Overall, I think (and the community agrees) that it is a good change because it increases the overall effect of player skill in the game. Understanding this is key in understanding the next part, so let's go over it in some more detail. If you play flawlessly and don't take damage for the floor, you should have a good chance to be rewarded with some sort of power level increase.
It's a good thing to allow skilled players to be able use their skill to differentiate themselves from unskilled players, and it is easy to see how this DD concept directly applies to a seeded race. But it also applies to unseeded racing and single-player speedrunning as well: over the course of ALL seeds, we want to slide the game towards a model where players who mess up are punished and players who play flawlessly are rewarded. (It's important to note that I deliberately chose the word "slide", since obviously there are still plenty of situations outside of Devil Deals in which players can be rewarded for making mistakes.)
Moving on from this, we can examine the health situation. In the Jud6s mod, we consider Judas to be a really skilled based character - you have to (potentially) play flawlessly for the entire course of Basement 1 and Basement 2 in order to be guaranteed the all-important power level increase from the B2 DD. In general, this is a really good game mechanic - players who consistently lose their half soul heart, on average, will lose seeded races, lose unseeded races, and get low Jud9s times on the leaderboard.
But what about other characters who are not guaranteed a DD based on their initial health? Now, this game mechanic doesn't apply at all. So by fixing this, from a game design perspective, we are artificially infusing more situations in the game where a skilled racer can differentiate themselves from an unskilled one.
I think that's essentially the motivation behind the change in a nutshell.
The Afterbirth+ version of Jud6s is considered in alpha and is located here: http://steamcommunity.com/workshop/filedetails/?id=842735484
The change log is located here: https://github.com/Zamiell/jud6s/blob/master/docs/version_history.md
From reading this thread, it looks like there are 7 votes for and 4 votes against, so I went ahead and changed the health on some characters in the AB+ version. For Blue Baby, he now has 3.5 soul hearts instead of 3. For Azazel, I gave him the same health as Judas.
Why change the health on Azazel in such a weird way? Well, here's my reasoning.
In general, in Jud6s, as you may or may not know, I try to make the minimum change to accomplish a goal. For example, if an enemy explodes on Isaac automatically when he enters the room, I move the enemy 1 tile away when I could really move them 2 tiles away just to be safe.
So how does that relate to Azazel? Well, we could (arbitrarily) quantify Azazel's overall power level numerically as a "50" on AB+ vanilla. We want to give him a guaranteed DD, because it makes the game more skill based. But doing so would increase his power level from 50 to something like 70, especially because it gives him an extra black heart nuke to use for room clear.
So, going from 50 to 70 is a big jump. How can we minimize that? Well, we could nerf him by removing the black hearts and giving him Judas' health. You could say that this version of Azazel would be a 60 - still stronger than with having 3 black hearts, but not as good as 3.5 black hearts.
Well, 60 is closer to 50 than 70 is, so this kind of change follows from the "minimum change" principle. It also reinforces the skill in dodging - Azazel is really powerful, but you really can't be sloppy with your dodging or you will straight up die.
Of course, I should say that I'm not committed to this change and maybe setting him to 3.5 black hearts might be better. But that was my train of thought.
Finally, I've updated the Jud6s website to contain a list of the 4 official codified goals of the mod, since I've seen some confusion as to what they are, and it might help to have that laid out explicitly.
(Note that the website is still not updated with all the AB+ stuff. I will do that later when the mod is not in "alpha" anymore.)
In reply to the Azazel health change:
I think the change to Judas style health is too drastic. It also nerfs the ability to USE your black heart and rely on getting a health up in the boss to take your DD. Or to just play recklessly at all in the first two floors (fuck it, you'll probably get B2 Krampus anyways Kappa). But as you said, adding an additional black heart seems like it's too much of a buff.
Alternatively, what about giving him his 3 black hearts and .5 soul heart?
or even still, giving him 2 black hearts, a soul heart, and half a black heart (from left to right)?
This way, he could still use his black heart as a nuke, but it would still be at risk of losing his deal. This is without changing his overall health structure too drastically.
In reply to everything else: BloodTrail
