blitz silence and channels

If it is a true "channel" where the champ cannot do anything else for the duration, yes. So if will work on Kat's or Malz's ult but not, for example, Fiddlesticks' damage aura after he teleports.

Yes it does. Channeling a spells means the spell is being casted for a duration. Using a silence on the user will cancel the channel. Heres a list of champions with channeled abilities: katarina, fiddlesticks, malzahar, rek'sai, anivia, galio, janna, karthus, master yi, pantheon, nunu, ezreal, shen, velkoz, zac, varus, twisted fate, sion(not his ult), vi (not ult), xerath, and warwick. I might be missing some.