Basically, the .txi files assigned to the lightsaber blade textures make any darkness in the texture appear transparent, specifically, the "blending additive" line in the .txi file. To change that and make a black lightsaber:
1) Find the blade color you want to change, for example w_lsabregold01
2) Write the texture to TGA using Kotor Tool and TXI converter, or create a new TGA file with the same dimensions and name.
3) Create a new text file and open with notepad. Type in "decal 1" without the quotes.
4) Save and rename the file, along with the extension, to "w_lsabregold01.txi"
5) The game will now recognize transparency in the .tga file as, well, transparency, and black, as, well, black.
6) Retexture at will.
Oh and BTW, yellow = yelo
EDIT - Got a small screenie up, whipped up the texture in about 3 mins. Far from refined, just demonstrating that it can be done:
http://img10.imageshack.us/img10/4484/blacklightsaberglow.jpg)
http://img10.imageshack.us/my.php?loc=img10&image=blacklightsaberglow2.jpg)