Back in BG 1 and 2 the Bhaalspawn were ever race, anyone and everyone could be a Bhaalspawn. They did not have dragonborn back then but they'd have had them be a Bhaalspawn too.
Due to that lore wise the dragonborn makes sense.
Yes, but these Bhaalspawn, if I understood correctly, were born from a mortal, impregnated by Bhaal. So they could indeed be any race, from mother's side.
But in the BG3 time all the Bhaalspawn were dead. Bhaal resurrected Sarevok, who spawned Helena and Orin. But the dark urge had no mother but was created solely from Bhaal's essence. So I think it should have old mortal Bhaal's race per default.
