could be wrong but dragonborn dont have dragon blood, they have the SOUL of a dragon
Same diff.
Anyway the dragonborns mother went out for a drink one night, met a strange man (akatosh) they flirted a bit, had some wine one thing let to another and before you knew it they were having a wild night of passion. The next morning she asked him if he wanted anything for breakfast and he said
"I'll need about three fifty"
And that's when she noticed he wasn't a hot looking Nord at all but in fact a gigantic dragon who could barely fit into her house. She kicked him out and the dragonborn never met his/her father.