Ouch...

Why do we say ouch or ow in indication of pain? Why not 'blark' or 'frooth'?

Apparently nobody knows. You could, and I'm guessing others have, postulated as to some sort of evolutionary tick that has manifested itself in that exclaimation, but apparently nobody knows.

It is slightly culturally varied in that some different countries have slightly different expressions of pain but they are more or less the same single syllable sounds. I believe Chinese people are one of the few peoples that have a double syllable expression of pain but it is still along the same lines.