Pikablu is the name that the internet fandom had come up with for Marill around early 1999 before his official English name had came out. A Japanese picture of Marill was leaked somewhere around that time thus making Marill the 3rd official Johto pokemon to be known about after Ho-oh & Togepi. Unlike those two though Marill hadn't gotten an appearance in the anime
yet (as we were in mid-Kanto by that point in the anime at the time), so
in a sense he was the most mysterious out of the 3. Thus wild speculation had occurred and since he hadn't made a debut appearance in the anime yet the fandom had absolutely nothing to go on at all but his physical appearance from that leaked picture of him. Because Marill is a mouse pokemon and he physically looks like a fat, blue, Pikachu that's how he got his name Pikablu from the fandom. Many months later during the Fall of 1999 promotion was going on for Pokemon: The First Movie. I remember seeing a new picture of Marill (alongside some other new pokemon) in either my October or November Disney Adventures magazine. The article of that magazine was about how there were 3 brand new pokemon! Two of them were going to be seen in a short before Pokemon: The First Movie, and one of them was going to be seen in the opening of that movie. Those 3 new pokemon were of course: Marill, Snubbull, & Donphan, and it was in
that article that had revealed Marill's official English name for the very first time. Lol, I remember as a kid that I was so used to calling him Pikablu all the time for sooo many months prior to that Disney Adventures magazine coming out that I had actually thought that
the magazine had made a mistake when they had called him Marill

.