I'd argue that the closest thing Transformers collectors have to Blue Snags is the figure that fans have designated "Bumper." Bumper is a vintage minibot that turns into yellow compact car; he has the same transformation style as Bumblebee and Cliffjumper, but with a completely unique body and face...and, here's the kicker, was accidentally sold in Cliffjumper packages during the 1980s. His name is really an abbreviated portmanteau "Bumblejumper" after the two other minibots. It is not unusual to see Bumpers sell for over $100 on eBay, even in opened or used condition.
I was lucky enough to have one as a kid. I still remember my dad taking me to Toys 'R' Us, and getting confused as to why the yellow car was in the package with the red robot picture. It was lucky for me, but not so lucky for "Bumper"...which now has detachable limbs and a removable head. These were not original features...but I was a kid, and toys were to be played with.
As the wiki link above notes, Bumper was retconned into the TF Universe in the comics series, where he was killed before ever even reaching Earth...but he has yet to be an officially recognized Transformer by Hasbro.
Should we start a petition for a Bumper DLC for Fall of Cybertron?
I'd also add two runners-up:
Red Bumblebee requires an addendum to my anecdote above. When I unwittingly bought one of the most elusive Transformers ever made, I also acquired a Red Bumblebee. My Dad had noticed that the packaging for the yellow robot featured a red car. We assumed the toys must have been switched (who'd of thunk otherwise?), so we bought both thinking we were covering our bases. In reality, Hasbro had made a series of Bumblebees in red plastic and just put them in the regular Bumblebee blister packs. I was, without knowing it, making the best toy run I would ever make in my entire life. (Incidentally, my Red Bumblebee is in even worse condition than my "Bumper.") R.B. does not typically command the same monetary value as Bumper, but he's still a rarity.
Often confused with figures like Bumper and Hubcap, Yellow Cliffjumper was (as is probably obvious) a Cliffjumper figure that used yellow plastic instead of red. He's pretty much identical in every way but color. He was sold as pack-in for a Transformers storybook cassette. Alas, I do not have a Yellow Cliffjumper, so I have no anecdote to provide here. There are also some rare Yellow Cliffjumpers that made it on actual Cliffjumper cards.
Of course, I'm talking with an American bias here. There were other color variants of these characters released in the foreign retail, all of which fetch high prices on the secondary market...but I'd say that "Bumper" is the most well-known by Transformers fans and most widely desired. But keep your eyes peeled. Because "Bumper" doesn't have an official name, you might stumble upon a seller who doesn't know what they have (I'll occasionally find eBay sellers listing any of the three Transformers above with incorrect titles, which can be an expensive mistake for them, but a lucky find for a collector).