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).