Round the Block by Fabjance is an oldie but a goodie. Simple in execution, but magical in its thinking. A sucker effect with an unusual ending.
The performer displays an orange block which he covers with a black cover. When the cover is removed, it is seen that the orange block has changed color to blue.
The cover is replaced and when removed, the block is seen to be orange again.
Of course the audience catches on pretty quickly, that the performer is simply turning the block to show a different color, etc. Finally the magic happens when the cover is removed to reveal a large blue BALL; the cover is otherwise empty.
One of those classics from Fabjance. Ca. 1970's, with original instructions. One side of the cover shows some wear, see photo, otherwise very good.