I know you are using GIMP and not photoshop but that's still no excuse, this tile is just plain ugly.
plus you didn't really give the neighbours anything to blend from.
More is required than just over use of the smudge tool, sorry.
This tile is from Newbies 10: Practice makes... black lines on the edges of your tile
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bleh!
Hey<i>!</i>
Why would use of The Gimp over Photoshop cause a bad blend?
All we ask...
All we ask is that you at least blend the borders a bit better :-)