Blk back will make your aquascaping and decor standout the most, if that is what you want the the look to be.
But I have also seen some people who have used the "asian 'driftwood / planted" background on their tanks... in combination with their own driftwodd/plants/rocks/etc. aquascaping layout, and this looks very good as well.
Of course, with the use of the same or similar decor as posted on the actual background scenery... this is the only way it can naturally blend in with everything else.
Try different variations and layouts and see what you like best. Trial & Error... it's all up to you really. Good luck!
ya know, i think this is what I need to do for my 300g. My silver dollars would stand out so much better w/ a black background, especially since they hang in the back a lot!
Unless someone is doing a nice 3d background the only backgrounds I care for are black. I paint the back of my tanks black, either that or use the adhesive type black backgrounds. I even painted the back of my wet/dry black.