In the screen below, you will see that there are three special blocks on the playing field. See the long, narrow box below the playing field? This is your special block inventory. The block inside of the red box is your next block available for use. By pressing "1" on your keyboard (which would be you), you will use your first available block on yourself. In this case, it's the b block. If you were to press keys 2 - 6, you would be using your first available block on one of the other players (either an opponent or a teammate). To illustrate its effect, I've used the block on myself.
The three special blocks that were on the playing field are now gone, thanks to the effect of the b block. My special blocks inventory has moved onto its next special block for use: the c block.