On most Wikis, a link to an existing entry is in blue, and a link to a non-existing one in red. If a particularly interesting item is linked a lot, and yet no one actually creates the page to go with it, the text remains red.
Then someone does create the page, and the resulting change in colour scheme makes the world feel slightly better. When this gets out of hand, you get an All-Blue Entry, and when it outspills the wiki environment, you've got Blue Fever.
Compare Rainbow Speak.
For the Half-Life expansion, see Half-Life: Blue Shift.