The black part needs to be really black.
Make sure the black part is 0,0,0 black (or a little bit more is also possible, i think) If the black is 15,15,15 the overlay flag won't work.
You can import it like said.
Then use this function:

action maketransparent
{
my.overlay = on; // the black part will be invisible
}

You can also just set the overlay flag on at object properties.

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It is also possible (i don't know what you want to do with it) to import it as a sprite (just a flat drawing actually, if you want to use it horizon, for example) then go to object--> load entity.. and choose sprite file and load you tga.
then you can set the overlay flag on.

I hope this will help!

(if you do what vlau said, the entire texture will be 50% transparent)