Building on that I have a question, or maybe two... If you have a bird that is a mix of two Ameraucanas (let's say a Blue and a Black), is it still an Ameraucana or an Easter Egger? I would think Easter Egger but maybe not? Would the 'future generations' of those hypothetical Blue/Black mix birds always be considered EEs, or could you somehow breed a pure Black or pure Blue Ameraucana back from them? I Hope those aren't ridiculously obvious questions, sorry if they are.
Breeding a Black to a Blue isn't really crossing varieties. They are both technically the same color/pattern (extended black), the Blue just happens to have a single dose of a dilute gene that turns black coloring into 'blue'. The Blue gene only affects black coloring, not the overall pattern.
Same thing with Wheaten and Blue Wheaten. Both are the same color/pattern so can be bred together.