If it's on one line, you could also make the text into a m. clip symbol, then make another that'll be a mask for it - black square the height of your text. Use a tween for the mask to make it expand to cover the text. Put both on the stage w/ the mask m. clip in a layer above text & set it to be a mask. [Tweak the frame rate of the whole movie to make it faster or slower.]
P.S. You can also do a search to get many other ideas - they're out there!