So you got a black edge to blend into... but instead of breaking it up, you drew a window in a black box, giving 2 surrounding tiles the same problem you had: all black and nothing to build off of.

Take some time and really figure out what you want to do with your tile... even if the guy to your left only gives you 15 pixels of black. There's an infinate number of things you can do with your tile to help surrounding tiles come up with something really neat to look at.