I have 2 purple-blue-ish marks on the top of my tongue, on each side, my dentist told me that its nothing to be concerned about, should i be worried?

Do you.....: Do you trust your dentist? If you do, then you should trust his\her judgement and advice. If you don't, find another dentist whom you do. Did you ask what the marks actually were? If not, then please do as this should allay your fears. My guess would be small varicose veins at or near the surface.

Nope: If your dentist said not to worry, you are 99% safe. If you want a second opinion, that's completely fine, too. Then you should see an oral surgeon or oral medicine specialist. Small purple-bluish marks on the tongue are almost always blood vessels near the surface...similar to varicose veins under the skin. Very common and harmless. But, if you want to be sure, get a second (or third) opinion.