There are a lot of activities you can still do with your dog, just not the show ring.
My 15wk old Lexi looks just like you describe, and the jury is still out on what her Dad was exactly. When I got the call, I was told Lexi was Fylo's little sister (and they do share the same Mommy, and a spot on the chest) but she has a lab's muzzle and a white toe. Just put up a new pic and will get a body shot on here soon...
Fylo is a rescue from my old neighbor, a typical BYB.
I think a lab found this guy's bitch left outside in a kennel, unsupervised. This litter was black, but Lexi's brothers had block heads and shorter muzzles compared to her so who knows, and who cares IMO, she needed a home.
Did the littermates have any markings? Or was the entire litter like yours?
Can't you ask this "breeder" about the possibility of your pup being a mix? Or does he just think black rotts are so common, it's not even worth discussing??
As far as how it will affect his rottenness it will depend on who Dad is.
There are many, many, good rotten mixes out there. Have fun & good luck with your baby.
Ailee, Fylo, & Lexi Marie