Black renders a bunch of genes invisible even if they are dominant on other colors. No way to tell by looking if a black bird has Pg, Co etc.
I suspect he is a Wyandotte cross or mix- he kind of looks like it and they are the most common rose combed large fowl. Wyandottes are a primarily eb- "partridge" breed.
In theory, if Snape carries eb/partridge there is potential to breed for birds pencilled all over in later generations with Mystique. However if he also has Co, there will be things like spangled, half moon laced, single laced, double laced and lots of intermediate patterns mixed in the results.
eb/partridge + Pg= pencilled, as in pencilled wyandotte.
eb/partridge + Co, Ml(melanotic) and Pg= single lace wyandotte. BUT, it is important for them to be pure for every single one of those genes. If they are not pure for all of them, it will result in the half moons, spangles, pencilling depending on what "combination" they have.
Mystique is not pencilled all over because she has something else in her but I don't know what it is. It may be as simple as being half birchen half eb/partridge.. or maybe she is pure birchen with Pg showing in the colored areas..or.. If they produce a chick with striped down with a marbled pattern on the head that would be instant proof both are eb/partridge carriers and pretty strong evidence for Snape being a wyandotte cross but it would also mean Mystique might have some wyandotte in her because partridge is practically nonexistent in hatchery stock NN.
Pg is very easily messed with.. or is really helpful in creating some patterns, depending on how one looks at it.