Mario, I have a Flametail Blenny that has scratched himself on the sand bed practically every day since I got him (not frantically mind you, likes he's trying to dislodge something - but lays down and scratches himself once on one side and then once on the other side). He was our first fish - we got him almost a year ago. He has never showed any white spots or other outward signs of any parasites, is very healthy (maybe a bit pudgy) and eats like a pig. I think he just likes scratching himself because it feels good (or maybe he has dry skin LOL). I could be wrong, but at least in my observation of this one fish, scratching doesn't always mean "ich".
Are you testing the amonia levels in your QT tank daily? Amonia can cause gill irritation - and if they are scratching on things near their head/upper body and/or "gasping" for air or breathing harder than normal, it could be amonia toxicy and not the parasites. Just a thought.