I think a respectful, HAND WRITTEN note to your mom would be best. Since she's 80, I think she'll appreciate more the time and effort you took into sharing your thoughts with her.

You need to be honest. Especially if you feel she already knows what's up. Tell her that times have changed and this type of relationship is more common than she might imagine. Tell her that this man treats you better than any white man you have ever been with, and that's more important than his race or color of his skin.

Tell her how happy he makes you, and doesn't she want you to be happy and loved?

Give her a big hug, tell her how happy you are that she cares enough to be worried about you, but that you're a big girl now and this man makes you very happy.

My 2 cents....
Kathy