What volume(s) determine a block trade? Do they show up on realtime streamers such as Etrade or Scottrade, or do you need a subscription service to see them?

Block trades are trades of 10,000 shares or more. That usually means institutions are doing the buying, but note that they also try to hide their tracks by buying in quantities less than 10,000. We use time and sales to see block trades, but E-trade tracks them as I recall. One place you can look for blocks at the end of the session is the NYSE website and the Nasdaq/AMEX website.