When will the blue in his rating disappear?  79? 80?  Later? 

With that, what's the highest a rating can be and still be blue?

I believe the rating will always change from blue to black when it changes from 79 to 80.

I have a guy who just went blue to black at 79. Does that mean he will get to 100 eventually with enough practice time on it?

Could be either. The change in color from blue to black in my experience represents approximately 20 points but is not consistent, just like the black to red represents approximately 7 but is inconsistent... typically give or take a point. Unless he has a very good WE, he definitely won't get to 100. Those last few % points are very hard to get to.