Erm, but not ALL supports are tanky. Some don't even need to be...
Lux, Morgana, Fiddlesticks and Zyra are some common examples. They are basically AP casters that can work vs certain match-ups on bottom lane. The only thing stopping other champions from doing so is their scalings. Sona used to have some crazy ones upon her release, same with Soraka. But they all got cut in half. So now there's no point in doing so. From other strange yet doable things - support Anivia can work, Kayle, Orianna or even Maokai or Malphite under certain circumstances.
People for some reason are afraid of playing so called non-supports as supports cuz they got addicted to metagame taken from tournaments and streams while it's not really the best approach. Remember that Leona wasn't designed to be a support but solo top, same with Blitzcrank. I still remember when I claimed on one forum that Leona could work really well with other champion on lane (not necessarily with AD Carry as at that time, ADC+support lanes just started to get popular at really high elos) and got bashed for it. Well, i was right in the end. And from more recent examples - Zyra.
If you think that something will work, by all means go for it. Who knows - maybe you will find something really good.
As a support you generally need to fulfill following conditions:
a) you can carry bot lane
b) you can be useful without many items
c) you can afford to buy wards
d) you can peel for your carries and offer some CC/other ways of saving them
That's it. If you were to build AP - so be it. As long as you can still do your job.
Also, I agree with Emeraldw - Thresh > Blitzcrank. Simply because he offers more in every aspect of the game as he was designed for bot lane and not top. More CC, FAR stronger utility when it comes to items (let's face it - Blitzcrank really loves having Muramana + Glacial Shroud - even back in S1 when Muramana didn't exist and solo lanes were rare sight, good Blitzcranks were scary stuff). TBH, as a support main, I would claim that in current metagame, good Thresh is basically free LP in massive amounts. There are reasons why some players had literally 15+ win streaks with him at platinum-diamond tiers.