No.
It can be black.
From what I understand, the blue gene doesn't just make the feathers produce less eumelanin, it changes how the melanin is distributed.
Instead of the pigment being spread evenly over the bird, it is clustered and stacked, leaving areas of black and areas of white. The overall effect is grey.
IE: the grey areas in this comic. But this is at a microscopic level so you can't see the stacked pigments.
View attachment 3938117
The soft edges of feathers and the soft hackle feathers have a different structure so the pigment is structured differently as well, allowing black to show. Additionally, the Pattern and Melanotic genes can increase the amount of black at the edge of the feathers. (Pg was introduced to many Ameraucanas from Andalusians but due to it's linkage to the pea comb and blue egg genes I believe Ml was not.)