She does look most like a Blue Cayuga. She is probably a mix that came out like that or a production quality Blue Runner that has lost its' Runner type after a few generations (someone else here had one that looked very similar and they quickly lose type in subsequent generations in it's not selected for). Anyway, it is a pretty easy color to get. I would probably call her a Blue Cayuga.
If you breed her to a Mallard drake, in theory you should get 50% Blues and 50% Black. You should really not get Blue Fawn because of the extended Black. Since you are crossing wild-type though to an extended Black based color, there will likely be some amount of the wild pattern showing through (although, generally not enough to really look like Blue Fawn). Some sources say Black is dominant, but it really does not appear to be. It darkens the feathers to mostly Black, but not entirely, hence some darkened Mallard pattern will show through. The Blue gene then will be inherited by 50% of the offspring and 50% won't get it, so basically you will end up with 50% Blacks and 50% Blues, some of which will have poor quality color from being only heterozygous extended Black (E/e).
