I just bought a blue roan from the same place I got my mare bred to a cromello stallion, and it just got me thinking about that colour you would get if you crossed a blue roan with a cromello? (not that I am going to do it, I was just curious)
Well the foal would be certain to inherit a cream gene for starters, since one parent would be cremello (2 cream genes).
Blue roans are black based horses with roaning, which may or may not affect the foal.
It would be easier to guess if one knew the parentage of the blue roan, to determine precicely what genes they were carrying. There are many black horses that carry red genes but it wont be obvious to the naked eye.
My guess (better experts can correct me) would be a smoky black of sorts, possibly with roaning.
Kind of depends on if the blue roan is a true blue roan or a bay roan, and then what the agouti status is of both horses, as well as the red/black status of the "blue" roan, as well as if the roan is heterozygous or homozygous for roan.
But basically you could possibly get smokey black, buckskin, brownskin or palomino for colors with or without roan on them.