1. I've always pictured Dennis Haysbert as being Blue Marvel in the MCU.
2. I've got no problems with Blue Marvel having a little grey hair at the temples, especially since that's the way he was designed by his creator Kevin Grevioux. I figure Kevin wanted him to look that way for a reason.
3. If they do introduce Blue Marvel into the MCU at any point, they could easily have him get his powers in the modern era (especially since people have been saying that his "forgotten hero" origin was taken by Carol Danvers in the MCU). It's not like they have to go the exact same route in the movies as they went in the comics (like for instance when Black Panther's father was NOT killed by Klaue in the MCU, but by someone else at that UN meeting).