Yes, using the Block command raises your shield. After a bit of time passes it drops and you need to spend a command turn re-activating it. From what I've seen (crappy shield and easy mobs), it does provide a fair bit of damage mitigation and feels alright as an active command--I just think that server lag currently prevents getting a good feel for how many other actions you can take in one Block duration.
Edit: As for "sentinel," I think it's just the name they're using to track shield levels right now. Shield does level, it just doesn't give its own class the way your sword does. As of now I am only level 2 shield so I do not know whether further levels will give shield commands, or whether these commands are accessible or useful to the DoMs which can technically use shields.
Edit: As for "sentinel," I think it's just the name they're using to track shield levels right now. Shield does level, it just doesn't give its own class the way your sword does. As of now I am only level 2 shield so I do not know whether further levels will give shield commands, or whether these commands are accessible or useful to the DoMs which can technically use shields.