In addition to what Bill had shared, "a hard smile" might also just mean "a hard to smile"...ex."she smiled a hard smile." And I may also agree to Sellymoo on her answer at "a hard blue". It may just only used more as an adjective. For example, "Has a big yellow sun and a hard blue sky." Yoyo, you may check on this link page for more idioms about "HARD" and their meanings: a HARD nut, a HARD nut to crack, a HARD time, HARD at, HARD of hearing, HARD-nosed, a HARD-on and many more.... http://idioms.thefreedictionary.com/hard

Hello, It may just mean a deep/dark blue or it can refer to the music style. In terms of 'hard smile', like Bill said it's a fake/forced smile. ^^ Hope someone can explain 'hard blue' better.

"a hard smile" means a force smile.