For a start, no flowers are really black and in fact black is not really a colour at all. So what we are talking about is those really deep red flowers and plants that look black.
Taking photographs of near black flowers is difficult. The problem is that really dark colours (very deep reds) absorb light, this tricks the light metre in cameras and the cameras in phones to give you the wrong exposure.
With cameras that have a spot metering system, or with a light metre, you can can take a light reading off the flower and adjust the camera to that reading.
With camera phones, which are so widely used, it is a different matter. You can use the touch to focus feature and that will activate a sort of spot metering. AND THEN if your camera phone has the feature, you can also use the slide up and down feature to darken the image.
Below we have a very deep red Aeonium.
In the image on the LEFT exposure is automatic. The image does not look as dark as it does to the eye.
The image on the RIGHT has been spot metered and exposure adjusted using the slider on iphone.
And of course you can use one of the many camera apps, that allows you to have even more control.
The problem with this is that when you alter exposures to darken photographs you often loose detail and clarity, so the best way to photograph really dark coloured flowers and foliage is under artificial light using a slow shutter speed.
You can find a good explanation of all of this at https://cameraharmony.com/photographing-dark-objects-how-to-do-it-right/