Maybe with Memblocks looping through each pixel and only changing certain pixels in the loop i.e. the bottom ones or looping through and changing the colour of every pixel except for the black and transparent ones.
It seems like it might be a lot easier to maybe remove the shadow from the sprite though and add it to another sprite on it's own and have it constrained/fixed to the main sprite so that it follows it when it moves around the screen. If the main sprite is animated meaning the shadow is animated too then just make sure that both sprites have animation applied and are in sync.