I have a dark round purplish spot on left side of tongue. whatcould it be?

Tongue lesion: This could be a simple bruise from biting your tongue. If it doesn't resolve within a few days, though, see an ENT for a possible biopsy.

Biopsy: A professional should evaluate the area. Usually if the area doesn't disappear in 2 weeks the area should be biopsied.

Blood blister?: It's most probably a blood blister from accidentally biting your tongue. However, if you want to be sure or if it's not gone in 2 weeks, see your general dentist or an oral surgeon just to be sure. Any suspicious lesion lasting more than 2 weeks should be examined and evaluated for possible biopsy.