The fungus Diaporthe samaricola produces perithecia embedded in the seed end of the samara (keys) of Ash.  Small black ostioles break through the surface to release the spores through a central pore.  These are visible as small black dots up to 0.4mm in diameter.

Neosetophoma samarorum produces even smaller black dots on these keys.

host/substrate

Please provide a photograph.  Some indication of size may also be helpful.  Note the host plant and substrate in your comments

On the samara (keys) of Ash

Winter and early spring.