I did a test like that some years ago while working on an article about flare and the film speed. The set-up consisted of the black box and four cards, white, grey, black, and mixed, with holes cut out of the middle to go over the black box opening.
Originally Posted by Bill Burk
I based the exposure on a grey card. I then made exposures of the four different set-ups. Being me, I then exposed a sensitometric strip to run with the black box test. I read the density from the area of the box opening and placed the density on the film curve.