The primary function of black out curtains is to stop any light from coming in to your room. You can use it for setting up an awesome movie theater where no light will come in or just to show off how fancy your house can look.
If you decide to go with crush velvet it might make it a little tougher considering noone will know the light is going to reflect off the curtains until light is actually shined on them. They shouldn't be used for bright areas because it will simply sparkle and shine too much especially if you want them room to get darker, which is a big reason people get big velvet curtains in the first place.
Velvet curtains are by default very thick and will stop light from coming in, but if you want to completely black out the room you might need to combine them with drapes, shades or blinds to get all the little nooks and crannies out of the window. If you plan on using it for a theatre this will most likely need to be used, especially if you plan on doing so during the day.
Decorate with sunflower decor or used moving blankets to move it.
