Black bricks with bio char.

I'd like to make black bricks to use in a keyhole garden. They'll obviously have to be able to stand the elements.

I'm hoping to avoid using dyes or paints of any kind. Does anyone know if adding biochar, fine coarse or otherwise, could result in a black (and most importantly sturdy) brick?

Are you looking to make unfired clay bricks ?

Bio char will burn up when fired.

I've incorporated it into clay test slabs. They were less durable in water, than clay alone. Probably suitable for interior walls. Ten percent charcoal produced very dark grey material.