It would be nice for the blend layers to support animations. This would look good for torch lighting effects.
It's done the way it is now for performance reasons. Blend layers need to manipulate every pixel under the blending images, and for animated blends this would be needed to be done at every screen draw, unlike just once like it's done now. Also the the work is done on the CPU instead of the GPU because java2d does not provide a way to write pixel shaders (custom shaders are a vague future plan even for javafx and we are long way from starting to use that because it was just recently open sourced). In code the change would be trivial: disabling the speed optimization and doing the drawing the dump way.
It could be worth it having a I-have-a-fast-computer-and-I-don't-care-if-the-rendering-is-expensive option for the client. That should be done with enabling full alpha transparency for fast enough systems (but can't be tied to the same heuristic. Even old systems can draw semitransparency fast when it's done properly on the GPU, but animated blends can't do it for the above mentioned reasons).
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Darn. And here I thought it would be something simple. :)