Here's what the Wiki says¹:

For a badfic to be considered Legendary, it must:

1. Be at least five years old.
2. Return a decent number of relevant Google hits from the past year, indicating that people are still talking about it. (You can select "Past year" in Google's Search Tools.) A "decent number" is at least five in the first two pages of results, i.e. 5 in 20, on the assumption that there are probably more hidden in the depths of the Internet. (This expectation may be lightened for very old fics, on a case by case basis.) Ideally, the search should be performed on the fic's title without quotation marks, since this demonstrates the fic's notoriety, but anything that gets the job done is okay. Hits on/related to a PPC mission to the fic don't count.
3. Be famous in its own right, not because of the author/sequel/prequel/fanart/whatever.
4. Be acknowledged as awful by a majority of PPCers with an opinion.

Bleepfic is considered to be, according to its article, "those badfics as painful and terrible to read as any Legendary Badfic, but not well-known enough by the community to be considered truly legendary." So, probably, bleepfic would also have to be voted on by the community, but it does not need Google Stats to prove that it is well-known.

Basically, legendary badfic has to be well-known for their badness (and solely that) and be acknowledged as awful enough to be legendary, as well as being old enough for the title. They are also often NSFW and NSFB, to boot.

¹Modified slightly because I don't know how to do those fancy "list within a list" things.

-kA