From Bulbapedia, the community-driven Pokémon encyclopedia.
ChoosenShipping (pronounced choh-sen; from Japanese 挑戦 chōsen; challenge, defiance ; Japanese: ブルシル BuruShiru, or シルブル ShiruBuru), one of the more popular ships from Pokémon Special, is the belief in romantic love between Blue and Silver. The name stems from the fact that they were the only two of the masked children to defy the Mask of Ice.
Blue and Silver grew close when they were raised as partners in the masked children; they seem to be much closer to each other than the pairs of Shum-Cart and Will-Karen do. Some slight evidence for this pairing exists in a bonus chapter entitled Escape!!, which showed the duo's escape from their captor. Early on, Blue removes Silver's mask and touches his face, causing him to blush. Later on, it's shown that Silver bought Blue's outfit for the Fire Red/Leaf Green story arc, and when Blue thinks of this, she too blushes.