Does this item (curtains) black out completely?

Very unlikely. If it were light blocking, or room-darkening, it would say so in the description. I wouldn’t buy these if a dark room is your goal.

I don’t think so. As @elbanditoroso says, they don’t say room darkening. They don’t even say thermal which would mean they were lined to keep out heat or cold which would also be room darkening.
There are curtains made specifically for room darkening or even black out curtains.
Example

I doubt it too. You need black out curtains if you want to completely block the light from outside. You can line these curtains with black out liner, or hide black out curtains behind these and draw them across when you want the room dark. Like what they do in hotels. One decorative curtain or side panel with a black out set behind.

I bought my aunt black out curtains at Bed Bath and Beyond and they work great.

Probably not. It would say in the description if they blocked out light. All it says is that they reduce sun glare.

Why not just line the window with aluminum foil. It worked well for me when I was growing weed.

I too agree with @elbanditorosom and @Valerie111.If it were light blocking it would be definitely mentioned in the description like this one.