Ok, either Im crazy and cant see well or the c maj 7 and c dominant 7 have the same keys highlighted. I think Im a open up 2 windows and look at them both at the same time
Try lookin' at it like this:
The C Maj 7 chord is all white keys (I'm only using this for this chord example, it is NOT a general rule of thumb) C-E-G-B
The C dom 7 chord has one black key C-E-G-
Bb (which is your black key)Again, we're just talkin' about this froma visual standpoint. For a hearing standpoint, go with what Elio stated.
I hope that helps a bit.