I have a dark red or kind of blackish spot in between 2 of my teeth one tooth has an old filling on it and was wonder what it may be?

Need more informatio. It could possibly be the result of a fistula and would need immediate dental attention. However, it could be what is called an amalgam tattoo. In that case it is nothing to worry about. That is why more information is required.
Lots. Could be food debris caught between teeth, damaged gum tissue, failing aged filling, cavity, or many other possibilities. Please call a pro, your dentist, for examination, diagnosis, and the appropriate treatment plan. This probably is not a diy project.