edit: There are no Yellow/Blue(girl) fics on AO3 and it's not even on the first page of Yellow results on FFN, so I'd say go for it! And for future reference to everyone else, if you're not certain if a ship counts as a rare pair, just do a search for them on AO3 and FFN. If you can't find a lot of fics for them, you should be good. :)