What is the intended effect ?

If the curtain is black enough for photometric experiments when it is stretched, it would be no worse when wavy (and probably better, as mre light will be lost in it)

If its to make some effect for peeps to see, then you might want it stretched, as waves would stand out on the otherwise uniform black background

Then I would have it stretched to look nice.
But the room would also be a light testing area.

If you want to stretch it vertically, fold its bottom end around a metal chain or something, so that it would weight down the bottom of the hanging curtain

If you want to stretch it vertically, fold its bottom end around a metal chain or something, so that it would weight down the bottom of the hanging curtain

And I would have to hide the chain really well, or it will ruin the effect.

Real theatre curtains have a heavy chain in the bottom. It is invisible as it is located in the bottom hem, however you can feel it if you pinch it there.

Sounds good.

I put up black fabric on all walls before in my band room it has a softer feel and dampen the noise I got it at Walmart

You might want to get some fire retardant fabric if you are covering all the walls in a room. This is especially important if it is a public place, but home insurance could be affected should there be a fire...

You might want to get some fire retardant fabric if you are covering all the walls in a room. This is especially important if it is a public place, but home insurance could be affected should there be a fire...

But will the fire retardant coating make it more reflective?

What about the trim? You should use glow in the dark white paint for a awesome effect

What about the trim? You should use glow in the dark white paint for a awesome effect

That will completely obliterate the effect! Another thing I could do, instead of trying to make the walls black, I could make the walls a bright black light paint.
Then install some black lights, and you have a nice effect!

The black curtain is used in theaters, where fire safety is at very high priority (after many incidents for as far as theater history goes). So it can be handled safely. Do the best practices for safety, as in any other room regardless of the curtains :

 - Avoid sources of ignition. No surge protectors, "cheepee" electrics, and other stuff that can randomly combust, except when you are testing them in a controlled way. Inspect all electrics used there for bad connections/damages. Keep high power lighting at safe distance from flammable stuff

 - Keep some clearance between any "big flammable things" and the exit from the room, so that any starting fire can't block the exit immediately

 - Keep fire extinguisher in the room

 - Specifically for the curtain : Make it removable, remove or fold aside when not in use, so that its in one place and not all over the wall most of the time. This will also preserve its blackness better

The black curtain is used in theaters, where fire safety is at very high priority (after many incidents for as far as theater history goes). So it can be handled safely. Do the best practices for safety, as in any other room regardless of the curtains :

 - Avoid sources of ignition. No surge protectors, "cheepee" electrics, and other stuff that can randomly combust, except when you are testing them in a controlled way. Inspect all electrics used there for bad connections/damages. Keep high power lighting at safe distance from flammable stuff

 - Keep some clearance between any "big flammable things" and the exit from the room, so that any starting fire can't block the exit immediately

 - Keep fire extinguisher in the room

 - Specifically for the curtain : Make it removable, remove or fold aside when not in use, so that its in one place and not all over the wall most of the time. This will also preserve its blackness better

The problem is, all of those items will reflect a lot of light, which means I would either have to paint them, hide them and memorize where they are, or not have them in the room.
And then there is the problem of what lighting to use.

All what items ? Extinguisher ?

All what items ? Extinguisher ?

The items talked about in the post, and the extinguisher would likely be painted red, not matte black, so it will ruin the effect.

Fabrics made for theater-cinema use have to be fire restive treated.So don't launder it or the treatment may be removed.And the curtain may be treated after it was made.These have to have a label on them stating that fact and that they meet the fire safety requirements.And of course-keep heat sources-such as lights away from it.You could still use fabric from a fabric shop-just again keep heat producing things well away from it.Note that fabric shops are disappearing-not as many folks are making things as they used to.A fabric-sewing shop closed out my way not long ago.A Hobby Lobby opened in its place-they have fabrics-but not the selection the fabric shop had.