That scene wasn’t about explaining how the Blob was back, it was about Magneto and how he was perceived by a number of other key mutant figures from his past. The Blob being repowered was a goof–nobody sat around worrying about how to address it, if they had, they’d have used another character entirely.
Comics are made by human beings. Human beings sometimes make mistakes. In the grand scheme of the universe, let alone the Marvel Universe, this is a relatively minor mistake–the kind that somebody at some point can address with a word balloon. Let’s not make more of it than it is.