From my perspective, I consider the "vacuum" as a physical medium. Pretty much alike as pneumatics, except, the pressure is a "negative"[1] one. Some car systems have the air compressor for actuating this or that, some less-sophisticated ones like our Blazers works on a "vacuum". The vacuum suction derivatives from a running engine operation. It constantly consumes (sucks) the air through an air intake, so that phenomenon could be used as a control medium[2]. Opening/closing valves or pushing/pulling actuators eliminates use of expensive, prone to damage, electronic equivalents, yet providing a necessary amount of force.

Dig through the forum for a diagram sketch of the vacuum lines. That should give You more understanding of the system.

P.S. Do not worry of asking the questions. That feeds the dinosaurs daily driving blazers - like us/me. I'll be glad to help. I've been helped, so I consider it my duty to help others.

[1] Well, according to physics it can't be negative, but below atmosphere pressure. "Negative" just states a "reverse flow".
[2] I wonder how about the supercharged engines

 

We may need to know the exact year and model and engine/transmission/transfercase setup to help.

 

Here is a thread I started in which I listed the size and lengths of hoses I used when I replaced all of mine. In it I included a pretty useful diagram of the connections.

https://blazerforum.com/forum/2nd-ge...replace-95425/

Only read this part if you are really interested in the nitty gritty of the system. LOL

As for how vacuum actuator works, the most technical view would be that what does the work of moving the actuator is the atmosphere that pushes against the side of the diaphragm opposite the side that is connected to the source of vacuum. For this to work, there will be valves in the in the control mechanism that will control to what one side of the diaphragm is connected; vacuum or atmosphere. There will often also be a return spring to bring the diaphragm back to the position opposite the one to which it goes when the vacuum is applied so that when vented, the actuator returns to its original position.

The vacuum reservoir is there for when the engine vacuum is not very low because the engine is under load and the the pressure difference between the intake manifold and atmosphere is reduced. Since no system is perfectly sealed, a constant "supply" of vacuum is needed.