I think OA is correct.
Total ways = 8P2 = 56
Ways when atleats oen of them is blue
BB - 2 cases (B1B2, B2B1)
BR - 4 cases (B1R1, B2R1, B1R2, B2R2)
RB - 4 cases
BY - 4 cases
YB - 4 cases
BG - 4 cases
GB - 4 cases
Total when atleast 1 is blue = 26
Total when none of them is blue = 30
Prob = 30/56 = 15/28
I think you forgot to take into account when one of them is blue. Question would be more clear if this would have been written "None of them is blue."
SAID BUSINESS SCHOOL, OXFORD - MBA CLASS OF 2008