Source? I did some rudimentary checking and what you're describing seems like revisionist history and/or conspiracy to me. I checked the news section from wikipedia on that date[1], and it's referred to as "Explosion on Deepwater Horizon drilling rig", with no references to BP (in the title, there are references in the body). The linked sources also do the same. The same lack of BP reference also applies to NPR which had stories from the AP[2] as well as new orleans public radio[3]. Maybe BP bribed all of them, but that seems hard to believe. Looking at the wikipedia page and/or news articles, you can find a less sinister explanation for why it wasn't called the BP oil spill: the drilling rig was owned and operated by Transocean, not BP.
[1] https://en.m.wikipedia.org/wiki/Portal:Current_events/April_...
[2] https://web.archive.org/web/20100421145719/https://www.npr.o...
[3] https://www.npr.org/templates/story/story.php?storyId=126183...