I have a black spot between teeth with foul taste but no pain what can be reason?

Gingivitis most likely. See dentist for cleaning and instruction on how to prevent this by brushing and flossing

You could a plaque build up of food that is decaying and causing an odor. It could be the start of early decay leading to a cavity. Either way you need to see a dentist for a cleaning and or a filling. Don't wait this will get worse.

You need to see your dentist for a proper evaluation and diagnosis! You could have a cavity and be suffering from caries, or you may have excessive dental calculus, or it may be an old silver amalgam restoration, or...

If it's on/in your teeth, probably cavity/cavities, an infection. If teeth are intact but there's a black triangle between teeth, probably due to gum recession, which in turn may be due to gum disease, an infection. Oral infections do not self-heal, and the longer you put off treatment the more the infection will spread. Please see a Dentist immediately.