I was just wondering, maybe I'm missing something and there's a simple solution to this;

I'm trying to make some bushes look nice;
By default the alpha of the texture is 0.9, which causes the leaves to have a distinct black margin, which kinda sucks when there are many tiny leaves;
Apparently the only way to remove that black outline is setting the alpha to 0; in that case the leaves look great, but too much of them is removed (some tight branches disappear, and some leaves are eroded too much).

Is there an optimum setting to remove the black outline without damaging the aspect of the texture too much?

It may be something different entirely, but I had a similar issue. I changed the blending mode to get better results. Maybe try MTECH_BLEND or MTECH_BLEND_LIGHT if you need them to cast shadows(if you aren't already using them).

Imo, there's probably something wrong with your texture then. A good way of having better outlines is to make the areas around your leaves (on the texture) more or less the same colour as your leaves.

Both solutions seem to be working great.

I will probably go for Tottel's one however, I think it should be more efficient even if a little compromising. Thanks a lot!

I'm doing it the way Tottel described. I think that's the standard anyway. Interpolation, mipmapped alpha, etc., all these things cause neighboring pixels to bleed in.. so it's a good idea to always put a bit of a border on your transparent areas or uv islands.