Obi-Wan seems to not have wanted to actually kill Viszla, so he only defended himself against Viszla's attacks. He even turned off the saber and clocked Viszla to end the fight. This is Obi-Wan, not Anakin, so he likely wanted to capture Viszla and find out exactly what was going on. Also, the darksaber is pure Lucas, so blame him. In a universe with planet destroying superlasers (whose beams converge and don't just pass on through each other) and guys shooting lightning from their fingertips, I don't have much trouble with a black bladed saber.