What is Pikablu, and how can I get him?
Solution 1:
As you can see from this page, Pikablu was a rumored pokemon, which wound up being Marill (#183), the evolution of Azurill (#298). You will see in the comments at the bottom:
"For months prior to the release of Pokémon Gold and Silver, Marill was often incorrectly referred to as "Pikablu", likely because of its slight resemblance to Pikachu. "
Solution 2:
Pikablu is not a real Pokemon. As the bulbapedia article explains, before the second generation of Pokemon games (Silver and Gold) was released, images of Marill were released and fans called it Pikablu.