Not knowing for sure, but wouldn't it be easier to use a macro as an enlarging lens?
You're not going to find many fast enlarging lenses either, although 2.8 seems to be pretty quick for both.
Anyway, the 50/2.8 Pentax FA lens has eight blades.
With the Nikon system you'll use either the 55, 60 or 105 Micro Nikkors or an aftermarket lens