I've done the black "wash" over the blue. In reality, the black acrylic KraftenPaent reactivated the blue and essentially replaced it on contact. It took several, several, many, many tries and retries rubbing back, adding blue, adding black, adding blue, spotting mistakes, making them worse, etc ad nauseam before settling on something that looks amazing and photographs terribly.
It's a blue/grey finish that I think will look really really nice so long as the wipe on poly doesn't destroy it. I might buy some spray laquer actually.