These foundation testers are available for dark skin.
Guyana, Fiori, Cosmos, Zinfandel, Samba, Havana
On the left you see some examples of models, so you can see if your complexion comes close to one of these photos. It doesn't matter if you don't have the exact same skin colour, as long as your complexion comes close.
Put some of the powder on the palm of your hand and test the colours on your jawline. If the colour blends smoothly without having a visible edge, you've found the right colour. You can also see it well if you apply foundation to one side of your face.