Is it an adhesive backed paper/plastic? If so, you can scrape it off and use some adhesive remover for the remnants. Be sure to cover the top of your tank so you don't contaminate it.
If you are able to slide the tank forward a bit so you can work behind it, mask off the parts you don't want painted and then brush on a few coats (drying in between coats) of black latex or enamel.
My skimmer hangs on the back so I moved it to the front of the tank. That allowed me to brush on a black enamel. It's possible to change the color. It just might be labor intensive depending on how your tank is set up and how the blue is currently applied.