Help me, im an idiot newbie!

Hey all im a bit of a newb, my computer broke and i lost everything and havent used flash for a month, i cam back, and i cant do a motion tween. I want to make a grey square gradually become a black square.

When i do the first frame, i make a square, convert to symbol, then i go a few frames later and insert blank keyframe, and change the symbol to a black, but then the first symbol in frame one is black too!

Ive done this before, ive just forgotten, im an idiot!

HELP!

Matt