Q:

Does Blitzcrank's ultimate silence or stop channeling spells?

I have seen this phenomenon not only once that when Blitzcrank runs into a TF, he silences for a short duration, stopping channeling for example.
Is this true, or only an assumption on my part?

A:

Blitzcranks Ultimate is a Silence, however it's duration is only 0.5 Seconds which is why it's usually used as an Interrupt (or for damage) rather than as a silence effect that prevents casting new spells. 
The fact that it's AOE and instant makes it a great counter to spells like Katarina Ultimate or Teleport and the main reason why it's a silence is because before the Kassadin rework there was no specific spell effect for Channel interrupts. The only thing that existed back then were hard CC or small knockups/knockbacks.

