I'm planning on making a cake frosted in white buttercream with black buttercream decorations. I'm afraid the color from the black is going to stain the white or leak into the white. Does that ever happen? Would refrigerating it help it, or hurt it, or make any difference in helping the black not leak onto the white?

Add meringue powder to your recipe. It helps the icing crust so that the color sets better.

referigerating it could make it worse. COndensation gets on the cake; the water droplets run, pulling the black color with it.

I've done black on white with no problems. Let the icing crust, then add the black decors.

I have a wedding cake in my photos done in non crusting buttercream, white with black scrolling on the bottom...I refrigerated it before delivery...no problems (except my husband sticking his thumb in the scrolling when he delivered it for me...says he's never delivering a buttercream cake for me again. Only fondant. haha.)