What size and shape of breath weapon do Sea, Sky, and Underworld Dragonbloods have if they take Breath of the Dragon? It states a size for if the breath weapon is a cone or line, but doesn't account for the burst that these three should have.
These dragons haven't been remastered yet. And Dragonblood is a heritage of the remaster, so these breath weapons aren't available for now.
The other point is that most of players' breath weapons lost their shape variety in remaster too. For example the remastered Dragon Breath is cone only now, Dragon's Rage Breath is also cone only too and the Dragon Form breaths are cone only.
So probably the remastered versions of these dragons' breath weapons for players would be cone too.
Anyway if your GM doesn't mind to do a minor homebrew IMO you these weapons would become something like:
The issue still existed when it was a Kobold feat called Kobold Breath in Pre-Remaster, and this seems to imply the Imperial and Primal (group, not Tradition) dragons are acknowledged options even if they fail to address all interactions.
Also, you're a little off about Dragon Form.
Dragon Breath [two-actions] You exhale deadly magical energy in an area, dealing 10d6 damage to each creature in the area with a basic save against your spell DC. The shape, damage type, and save type match that of your chosen dragon's breath. If the chosen dragon's breath can deal more than one type of damage, choose one when you cast dragon form. The shape is a 30-foot cone or a 100- foot line.
This continues to fail to account for the three Imperial Dragons with burst-shaped breath weapons. You can turn into a Bog Dragon (remastered Black Dragon), and get your line of acid just fine, but they continue to forget that these three Imperial dragons have neither a cone, nor a line as their breath weapon shape. Perhaps the writers keep confusing them with the 1E versions where they either had a cone (Sky Dragon), a line (Underworld Dragon), or both (Sea Dragon, a cone of steam that dealt fire damage, or a pressurised line of water that dealt bludgeoning).
You're confusing the Underworld Dragon, with the Umbral Dragon there Elric200. Underworld Dragons have always been connected to Fire (and originally even had adamantine claws back in PF1E).
Well, Draconic Codex is out to subscribers now and it clarified how to treat this trio of Imperial dragons breath weapons when using PC options in, IMNSHO, the least interesting way by treating them all as cones. Immediately houseruling that Sea & Underworld have lines not cones for PC options or maybe I'll just say f it, I'm in vehement disagreement with the way it was decided in the official rules anyway so I'll houserule it to how I think it should be, 10' burst within 20' for the basic tier like from Breath of the Dragon/the reintroduced Kobold Breath, upgrading to 20' burst within 40' with the respective upgrade feats, and I'll change the Kobold Momentum feat's prerequisite to be "have a breath weapon" so dragonblooded kobolds can take it as well, using BotD as the qualifier instead.
The major difference between Remastered Kobold Breath and Breath of the Dragon is that the former when upgraded has higher burst potential by going to a bigger die size than Breath of the Dragon can reach in exchange for having to wait an hour before using the breath weapon again and it can't recharge early with a certain feat, whereas BotD has an upgraded die at no cost but can do the larger area all the live-long day after upgrading with only the normal 1d4 round(s) recharge time. Kobold Breath's persistent damage upgrade also has a higher minimum damage with the same maximum so on average it's just a bit stronger at the cost of not creating difficult terrain and not applying to those who only normal fail their save.