ive used contact paper on the outside of the tank. more recently i bought some styrofoam at lowes. super cheap. i wrapped that in contact paper and very discretely taped it to the tank. i think i like the velcro idea better though.

ive seen people use all sorts of things to "black" out a tank. ive seen pennies, $2 bills, cds, cd covers, kids artwork, photos, etc..