I'm also on the lookout for a straight black SDL->PS/2 cable, however they seem to be rare as hens teeth so in the meantime I came up with a workable substitute.
I ended up sacrificing one of my coiled cables. I clipped the straight segments from either end and stripped off the outer cover and the inner wire insulation from both segments. Some wire twisting and a bit of electrical tape later I had a short straight SDL->PS/2 cable, which I coated with black Plasti Dip spray. I mated this with black PS/2 extension cable and voila! I hid the ugly section where the joint was covered with electrical tape by taping it to the back of the keyboard.
Unfortunately my wife is out of town with the camera until Sunday so I can't post any pics until then.
OK, here's a pic of the underside of the board:
(http://www.soybomb.com/ratpfink/keyboards/BlackSpaceSaver2-sm.jpg)
I ditched the tape and re-attached it to the underside of the keyboard with zip ties. Not the most elegant solution but I always keep the risers up so this isn't a big problem for me.
Here's how it looks on the desk:
(http://www.soybomb.com/ratpfink/keyboards/BlackSpaceSaver3-sm.jpg)
I guess you can't really see the cable in this pic, although that's kinda the point of this mod...