yeah if you're going for a black pearl star start as early as possible...
Every move you make should be with the intent of making a flower. If you're in marathon mode there's no time limit so don't worry about sitting and thinking about a move for a while.
Also if you create a regular flower around a star a new star will drop down from above, it's a lot easier making a flower if you can rotate the pieces of the flower around.
I can usually get a black pearl by the end of level 1... I've had enough black pearls to make a flower out of them once but I a bomb came and I lost the game before I had a chance to assemble them