All the Mallard derivatives, eg Campbell, Runners, Pekins, Saxony, etc, can interbreed successfully producing fertile offspring. Its only a Muscovy x Mallard derivative that produces infertile offspring.
A black duck you could use with the black Runner is a Cayuga. Its been used in the past to create black Runners. You then have to vigourously cross back to a upright standing Runner to get the stance and fineness back.
I'll leave the experts to explain line breeding. But in a word, yes, you can breed Mother to Son and even brother to sister for at least the next generation before new blood may need to be sourced. In terms of creating mutants, nature takes its course. Mutant ducklings usually don't hatch or you'll find they don't thrive.