Woah! I'm so late you have moved onto the next critter.
This conversion is brilliant. But there are a couple of things I'd like to cast a topic resurrection spell on, because, while you have great logic here, you are up against Jeff Grubb's "rule one" of Spelljammer:
Concordance of Arcane Space: Foreword said:
Everything you know about space is wrong.
I've brought this up because a fairly significant amount of Concordance of Arcane Space is all about saying how wildspace and the phlogiston are
not like space as we know it. Essentially, if you do things "right" then you are wrong, but if you do them "wrong" (at least if you "wrong" in the way that Jeff Grubb tells you to be "wrong") then you are right.
(Actually, I think that one of the biggest problems with Spelljammer is that some people just can't accept things being "wrong" and spend ages trying to find a way to make them "right". You can see tons of posts and threads written by people that don't like the phlogiston or binary gravity or some other feature. Other people might accept that things are "wrong" if it wasn't for the fact that the "wrong is right - but right is wrong" concept confuses them. Those people spend time asking questions to help "get" what Jeff Grubb was trying to sell. So there is always a level of background chatter about the funky universe rules in the SJ community. But that is getting off the point.

)
Maybe the simplest thing is to leave off the fire subtype (as odd as that seems) and give them immunity to cold. I'd give them immunity to fire, too, but would the phlogiston bit be an exception or some other untyped damage?
I know what you mean about spelljammer.org. They do have an epic spell that creates an air envelope around a ship to protect the crew from "cold and radiation" from wildspace, so I guess that's there would be cold damage. If Big Mac shows up, maybe he can tell us.
Cold is not a problem for any sort of critter in wildspace:
Temperature said:
Due to the activity of the planets, the spheres, and various gates to the plane of elemental fire, temperature in wildspace is not a problem for adventurers. The ambient temperature in most space is about the same as a moderate summer day in the termperate regions of most worlds.
Some crystal spheres however, have suprisingly higher or lower temperatures, and those should be noted on any star chart worth reading. A shell that is little more than an opening into the plane of elemental fire will have a higher temperature than one without any such openings. Krynnspace has a very low natural temperature (about 16 degrees farenheit) and is plagued by small, lethal clouds of ice particles.
The phlogiston maintains a similarly comfortable temperature, but with the added danger of explosive fire, as described above.
There are no seasons in the phlogistion or wildspace. Of course, as a ship approaches a star, the temperature increases. Landing on a body of fire creates problems similar to entering the elemental plane of fire. A water body could conceivably be cold enough to be nothing but a ball of ice.
Anyhoo. The main thing is that the fire subtype is generally not a problem for any SJ critter in wildspace. But if a blazozoid was in Krynnspace and flew into a
Cloud of Freezing Vapor it would probably be destroyed. I don't think that needs any specific rules.
As for radiation, I'm not sure that exists in the Spelljammer multiverse (and therefore the D&D multiverse). The closest thing I've seen to "radiation sickness" is what happens to anyone that associates with a radiant golem. IIRC, it is all very non-specific.
Starborn (Ex): A blazozoid can exist comfortably in the harsh conditions of space. A blazozoid does not have the vulnerability to cold that is standard for creatures with the Fire subtype, and it takes no damage from radiation, fire. Blazozoids are immune to pressure and decompression damage, so can move easily between the vacuum of space to the crushing depths of a star.
As well as Jeff Grubb making wildspace (mostly) warm, he also gave objects (above a certain size) in space a personal "air envelope" (Breathing in Space CoAS pages 11-13). So the pressure thing really only applies if a blazozoid wants to fly deep into a sun or a airworld. I'm not sure it needs to do either of those things.
Maybe this ability doesn't need to stay. But if it does, I think it has the wrong name (or rather the "right" name that makes it wrong for Spelljammer.) Another one of the "wrongnesses" of Spelljammer is that a star and a sun are not the same thing. Stars are much smaller objects than suns and don't have a fixed nature:
Those fantasy systems with stars in the night sky often have these stars mounted along the inside of the crystal sphere. The nature of the stars varies from sphere to sphere. Within some spheres the stars are small portholes looking out on the phlogiston, in some they are painted lights along the interior, in some they are great cities inhabited by alient creatures, and in others they are great bowls of fire held aloft by huge statues of forgotten gods.
In contrast to this a sun is a fire body that provides both heat and light within its sphere. And they are usually, but not always, at the centre of their sphere (instead of the inside of the sphere wall).
Pretty weird stuff. I'm not suprised that some people get confused by this stuff.
Is "I" next, or is it waiting for the epic thread?
I don't have the stats for "I", so unless someone can provide them, I'd say save them for sometime in the future when I can acquire those stats.
You are not going to find the stats for "I". One of the slightly frustrating things about Spelljammer is that there are lots of interesting ideas that are thrown down in isolation but never picked up again and expanded.
I have seen no Ispace and no adventue that uses the blazozoid. There is a lot of potential, and it would have been great to have seen something done with it, but it isn't going to happen unless someone brings back Spelljammer and allows the fans to make a list of loose ends that need tying up.
But last time SJ came back, it was more of a reboot than an expansion of what was before.
If you want to extrapolate some stats for "I" from another part of SJ, there is a world called
Garden in Realmspace that is also alive. That "liveworld" has tons of hit points and regenerates them.
"I" isn't said to be a liveworld, but it is so similar to the other liveworlds, that I'd treat it as one.
Liveworlds and starbeasts (planet sized creatures that are so big people live on some of them) are so big that in a way it is pretty meaningless to have stats for them. They are quite similar to gods in that they would be pretty indistructable. In fact, I suspect that if "I" found out about gods it would think that it was one (and that blazozoids were its avatars).
You could make hit points for gods that PCs might meet in Planescape, but you kind of need a different type of stats for them (and their avatars and clerics). I think that people would mostly need to know what mental powers "I" has in its own sphere (i.e. can it cast spells), how fast it can create blazozoids and how far away from the sun a blazozoid can be before it is unable to communicate with "I". That is all going to be guesswork.
PS: I'm a big fan of Spelljammer: Beyond the Moons, but the website has not had many changes recently. I've been told that there is a major behind the scenes overhaul of how the site works going on, which has meant that mundane changes have had to be held back, to allow the other work to continue. Keep watching the skies, as spelljammer.org may have some awesome changes soon.