Post a picture of her if you can, is she Lilac(aka Blue Fawn) or a different color? Not sure about breeding her to the black drake BUT I would breed her to the chocolate boy.
Lilac/Blue Fawn x Chocolate will give you some of both colors. My Blue Fawn hen bred to my Chocolate drake and hatched mostly 10 Blue Fawns & 2 Chocolates. 3 of the Blue Fawns were much lighter though and those are the ones I refer to as Lilacs.
I found this, maybe it will be of some help to you.
This is my a couple of the hens I kept this year from the clutch i mentioned above. A Blue fawn hen & what I would refer to as a Lilac hen, you can see she is a MUCH lighter version of the Blue Fawn. Their mother is a Blue Fawn & father was a Chocolate.
And a Blue Fawn next to some of the Chocolate Sisters.
My understanding is that lilac is two doses of blue ie silver + choc dilution (N/N, ch/ch or ch/-). Blue fawn is one dose of blue ie blue + choc dilution (N/n+, ch/ch or ch/-).
Lilac female to choc drake:-
If she is lilac ie N/N, ch/-, & he is choc (brown) ie n+/n+, ch/ch, then all ducklings should be blue fawn = N/n+, ch/ch (females = N/n+, ch/-)
Lilac female to black drake:-
Lilac (N/N, ch/-) female to black (A+/A+ or a/a, n+/n+, Ch+/Ch+) drake = 100% blue progeny because she will pass one dose of auto blue dilution (N/n+) to all progeny, hence blue, but, she will only pass the sex-linked recessive choc to male progeny who won't express the trait.
Patterning may occur in some progeny eg pied patterned?