Bernard and Lenina, at the beginning of the book, seem to be polar opposites in most things. Bernard is fronting himself as anti-social, a bit of a loner, and as having a disgust and repugnance for all of the group games, mindlessness, and empty relationships that are in his society. Lenina on the other hand is a social butterfly who loves group activities, being with different people all of the time, and enjoys the full benefits of the rather casual relationships of their society. Brooding Bernard mopes about, finding reasons to be gloomy, and taking pleasure in things, like being alone, that no one else does.
When the two of these people on the roof see a beautiful blue sky that surrounds them, the associate it with different things. Bernard's reaction:
"Bernard Marx drew a deep breath...'Isn't it beautiful!' His voice trembled a little."
He is struck by the magnificent scene, touched by its poetic majesty, and thrilled to the core by it in that sense. Lenina also finds it exciting, but for a different reason; she states, "Simply perfect for Obstacle Golf." So, she too is moved, but ties it not to beauty or aesthetics, but to the contact sports that are so popular in her society. To her, a blue sky means only good games, not something to be appreciated and enjoyed just because it's pretty. She is a product of their civilization for sure, trained to enjoy what they want her to enjoy, on their terms. Bernard, a bit of a wild card, has the audacity to take a breath of fresh air and notice a sky is beautiful for no other reason than its mere existence. It's an interesting comparison between these two characters; in the end, however, you will find that Bernard isn't much different than Lenina in many ways. I hope that helped; good luck!