What does your texture look like? If you have the shape in the diffuse channel with a black background you'll get a black edge due to the filtering and blurring. Try making your texture's background the same brown as the main part (in other words, the entire colour channel would just be brown, with the shape defined in the alpha channel only).
Hope that makes sense.
If not, please post a picture of your texture and also of its alpha channel.
Hope that makes sense.
If not, please post a picture of your texture and also of its alpha channel.