This is completely anecdotal evidence, and sadly I didn't log it. However, I recently gave Scax a cutting damagebuff tattoo. I don't remember the exact values of the damages and what not, but the damage before and after had an exact 10% difference. We both thought it was pretty nifty. This was with him using a standard swing attack and assault. He's a pureblade if I remember correctly.

Someone used me to test damage before and after getting the tattoo a while back. I can confirm that it indeed works, and is a 10% increase (assuming nothing else is giving offensive dmp buffs).

EDIT: Against players, at least.