In this youtube video, there's a silver hose connected to the intake tube at 0:03 just left of the prop rod . This hose appears to be missing on our Blazer, but the port it goes to is there. What is this hose for, and where can I get a replacement?

 

If you are talking about the vertical curved tube just to the left of the prop rod at time=0:03, that is the air inlet pre-heater tube that goes from the exhaust manifold to the air cleaner snorkel. The valve on top of the air cleaner snorkel (if still hooked up to it's vacuum switch) closes and draws hot air into the air cleaner when the engine is cold.

Need to measure the size that you need. Held on with hose clamps.
1.5" = Dorman 96020
1.75" = Dorman 96022
2.0" = Dorman 96024