There are many layers to this complex metaphor. Please read the following descriptions very carefully, otherwise the full meaning of the saying cannot be appreciated.
Blue represents boy. It is the color most often associated with clothing and toys for young males. Blue should not be confused with pink. At this point it should be completely clear that Blue is synonymous with male.
Crush refers to a feeling of attraction or infatuation. This should not be misunderstood as the feeling of excitement on Christmas morning or the love of a sports team. In this context Crush is amorous in every sense of the word and it only serves to describe romantic relationships between humans.
Now that we understand both Blue and Crush separately, we can attempt to combine the 2 words.
Blue + Crush = Blue Crush
If you're still with me we have effectively created an entirely new meaning that was not possible with either word on its own. Since we know that Blue means male and Crush means romantic attraction, then we come to the following conclusion:
A Blue Crush is when a male is attracted to another male.
What does this have to do with the 2002 film Blue Crush starring Kate Bosworth? Absolutely nothing. In fact, men were attracted to women in that movie (in theory).