Blitz negates all of Soraka's power because he doesn't need to grab the carry, he just needs to grab someone.
Once that person is silenced, exhausted, and ignited there is nothing Soraka can do. All the heals in the world won't save you from 4 seconds of CC.
Basically, he and Leona out CC and out damage her heals.
The one aggressive support that Soraka can shut down is Taric because she can just silence him when he appears. But, in the ever flowing, ever balanced cycle of bottom lane, Taric can poop on Blitz.
Taric sits in the line of fire and dares Blitz to pull him in. All it does is pull him closer to the carry who will now be stunned and shattered without any peels from Blitz.
If a Soraka wards the lane brush there should be almost no reason for anyone to ever be grabbed.
It is very, very easily avoidable and if Blitz tries to sit in a position to go for a hook in that brush most ADs can poke him without mercy.
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"What I'm saying is mind over matter only works when there's not much of a matter to mind. " - Lopen
The Cult of Personality.