How can I make "screens of black for grey" in Illustrator?

I need to prepare our company logo for printing on some envelopes. These are the instructions I was given:

Everything as two spot colors - Reflex Blue and 100% Black with screens of black for the greys

I've already take care of making sure my blue is the Reflex blue pantone in Illustrator... But what does it mean to use "100% black with screens of black for greys?"