What color do red and black make?
Red and Black make maroon.

Red and Black make maroon, #660000. That is the result in equal parts, mixing pigment rather than light - paint, ink and dye all behave this way.
Equal parts red and black · #660000
Change the ratio
Very little of a mix is ever half and half. Shifting the balance moves the result along the range between the two colours.
3 parts red, 1 part black · #b30000
Equal parts · #660000
1 part red, 3 parts black · #260000
Lighter and darker
With white added - a tint · #994d4d
With black added - a shade · #330000
Why it works
Adding black to red makes a shade. Black absorbs across the whole spectrum, so the result keeps its hue but loses brightness and some saturation, which is how red becomes maroon.
Frequently asked questions
What color do red and black make?
Red and Black make maroon, #660000. Mixed in equal parts that is the result you get with paint, ink or any other pigment.
How do I make the result lighter or darker?
Add white for a tint - here that gives brown. Add black for a shade, which gives black. Add white a little at a time, because it takes far less than people expect to shift a mix.
What happens if I use more red than black?
Three parts red to one part black gives maroon. Reversing it to one part red and three parts black gives black instead.
Does this work the same way on a screen?
No. Screens mix light additively, so red and green light make yellow there. These results are subtractive - paint, ink, dye and pigment - which is what people mean when they ask what two colours make.