Blue arrived at the party considerably later than the rest of the mountain lions. Besides not wanting to arrive together, as that would warrant ample suspicion, he had considered not going. He didn't want to get caught, and he still wasn't feeling up to being sociable.
However, after some contemplation, Blue decided to suck it up and get dressed up - he chose an old batman costume. Batman was always his favorite superhero - he was able to create his own superpowers and was actually just a normal (albeit, extremely rich) human being. When he was little, his parents told him that he could do whatever he wanted, and until the age of eight, his life's goal was to be Batman. He figured this was an appropriate costume choice, with the proper mask to hide his identity, like the real batman.
Blue finally arrived at the party, donning his mask, outfit, and cape. He noticed a familiar silhouette in the corner, wearing a mask he had seen before. That height and shape, combined with the mask, it had to be Danni. Blue thought that he had seen her wearing this in a dance performance last year. He went over to her, against the wall, and said, "This seems to be quite the social gathering."