The Marc train has stops at both BWI and Camden Yards, but on two different lines. You would probably need to take the Penn Line to Penn Station, then the Camden Line to Camden Yards.
The Light Rail Line directly goes from BWI Airport International Terminal lower level to Camden Yards. $1.60 each way or $3.20 round trip. Light Rail runs for 1 hour after the end of Orioles games or 11 pm, whichever is later.