Yes, you can send black or colored envelopes in the mail, provided that the color is well contrasted between the information written on the envelope and the color of the envelope itself. If you are mailing black envelopes, you need to write addresses in ink that's clearly visible.

What does a black envelope mean?

BLACK envelopes signify power and control. It's sometimes represented as a scary color, like for Halloween.

Do colored envelopes require extra postage?

You may use colored cards and envelopes for mailing (as well as different ink colors) if there is no interference with the reading of the information or postmark. First and foremost, the mailing recipient's information should be clear and legible.