This may sound odd, but I remember seeing an A-arm on a FSAE car that was tig welded with I believe a stainless filler rod and then black oxide dipped. The result was the tubing turned black, but the weld beads stayed a shiney silver. Does anyone have anyone have an idea of what type of filler rod and technique could be used to reproduce this effect? I think the tubing was chromoly, but not sure.

Thanks