How to create black background with iPhone?
 

Is it possible to take pics of blooming orchids with black background using iPhone?

Because I read that it is usually done using DSLR and external flash.

Using iPhone, not DSLR?

One thing, on the iPhone, I think HDR is selected by default, turn that off.

Have the bloom/subject of the photo in a brightly lit area, like a dining room table with lights shining on the bloom probably/maybe also enable the flash. Then have the rest of the area dark. Then have the camera close to the bloom and take the photo in such a way that the next nearest item is as far as possible away from the camera. IE, if your room is 10 feet x 20 feet, take the photo so the next wall is 20 feet away, not 10 feet.

Have a brightly lit subject very close to the camera and the next nearest item as far away as possible and dimly lit will/can make the far away item fade into blackness. The HDR setting on an iPhone specifically tries to counteract that tendency so you have to turn it off.

For HDR photography, the camera will take 2 or 3 photos one with so you can "see" the dark parts of the photo, one where you can really "see" the very light areas of the photo and one where you can see the main subject of the photo, then merge those 3 into one photo. What that means is that you can more detail in the photo than if the camera had just taken the one photo of the subject.

I'm planning to buy a kind of a black vinyl screen, framed as a painting, to use as those photographic panels used in photo sessions (ex)

Use black velvet, rather than vinyl, and place it well behind the subject. Velvet just "eats" shadows.

I thought about vinyl because velvet might add a certain amount of grey tone to the background. I also think, if velvet is placed "far" from the objective, that grey tone might not be noticeable. But to do that I'll need a large screen, which goes against my need for a small one.

Unless you get some sort of special non-reflective vinyl, I don't see that working very well. Velvet would be better, or maybe the material that Astronomy folks use to "flock" their astronomy gear.

I believe that using the correct angle would minimize reflections caused by a flash or window light. Just need to avoid vinyl folding when framed.
But I couldn't find vinyl so far so I might try velvet first.