Thanks - I'd like to make the centre black, that would also help the game flow from area to area (Metroid style) but I really struggle to make it blend well in the simplistic style I'm using.
Well as far as I've seen in 2D games, both modern and recents there are multiple options I can remember :
-Make a gradient : The problem with that is that if you go overboard with your gradient (too may tones) it won't look good with the limited palette of pixel art
-Make a border : Make a border just like the actual border you already have for the side around the edges
-Add detail and lessen it : Add just a little bit of texture to the brown, and you can go with less and less detail until the details fade to black, that's hard to actually explain, but it's mostly used for stone anyway
Also, I'd say that a black tile center shouldn't be black but really more greyish (probably dark pale blue).
Maybe some tiling experts will be able to determine which is better for your case