I would caution you to be absolutely certain of your parents' blood types before you say anything to them. It would be horrible if you "confront" them and then find out you are wrong about their blood types.
Also, how old are you and how old are they? This would be a different conversation if you are 60 and they are 85, than it would be if you are 15 and they are 38. (If you are in the younger range that I mentioned, please wait to discuss this until you are 18 or 21 or 25. A teenager 'confronting' his or her parents about something like this is not a logical or pretty sight.)
Please also keep in mind that although it's not in line with new thinking about adoption, some agencies do counsel the adoptive parents not to tell their child ... if you find out that you are adopted and were not told, your parents may have been acting in good faith and doing what they were told was best for you. Parents don't hide the fact that someone was adopted in order to hurt their child, but because they think that for one reason or another it will be better not to tell.
One more natural and gentle way to bring it up, if you feel a confrontation is necessary, is to tell your mom or dad that you are interested in doing a 23-and-Me style DNA test for fun, to see about geneology and hidden ancestors the family doesn't know about. If they know something you don't know about your background, they will probably mention it then.