91 Tbird factory to aftermarket radio wiring

I am trying to install a basic aftermarket stereo into my 91 base V8. Everything seems to be connecting correctly, but I have found there are 2 ground wires coming from the car's wiring (one black, one red.) Not sure if I join those 2 and then connect into the new radio ground wire, or if one gets capped and left off.
I think the red is power and black is ground. There is also a blue power I believe. I have a schoose or however you spell it aftermarket adapter that works well.
Thanks for the reply. I tested both red and black to ground (i know...weird...) but I wound up capping the red because since there was already a live power - i think it was the blue wire. Anyway - everything seems to work ok like that.