Margaret...
I believe what Stephen was suggesting was an alternative to using the history brush to physically brush on the effect of the Levels adjustment.
Once you've created the Levels adjustment and (in the history palette) clicked on Open and next to the Levels adjustment (to set the history source), instead of selecting the History brush and painting away, create a selection by whatever method is appropriate (lasso, quick mask, pen tool, creating a mask from one of the color channels, whatever) of the area(s) on the image on which the Levels effect is to be applied. Then use the Fill command (Option - Use: History) in order to fill the selection from the history state.
This alternative may or may not yield better results than the history brush. Once instance where it might save some time is if you've got a huge area to restore from history. Fill could be faster than manually brushing. Then again if it takes 30 minutes to make a good mask, then maybe using the history brush would be better. In any event this is just another tidbit to toss into your growing bag of tricks.
Regarding
"...make sure that black is the forground color and start painting with the history brush over the people."
I don't believe FG/BG colors are applicable to the history brush. It's my understanding that the history brush gets its "colors" from the history state selected in the history palette. Where painting with black
is applicable is if you're painting (usually with the airbrush) on the adjustment layer itself.
Black will "disable" (or mask) the effect of the adjustment on layer(s) below. In general painting on the adjustment layer in this type of situation is usually a safer and more flexible technique than using the history brush on the BG layer.
The history brush paints on the destination layer. In some cases that may be OK. Painting on the adjustment layer, however, leaves layers below in tact. You can modify the adjustment layer by adding black (to suppress more) or white (over black areas) to restore the adjustment effect -or- deleting the adjustment layer altogether and add a new one and start over.
Don't get me wrong. Knowing how (and when) to use the history brush and the history palette are powerful techniques. Like you, I was tickled when I finally connected the dots on using them. I'd like to add my appreciation for posting your discovery. As DJ and Ed noted, everybody benefits from tips like these.
May many more "lightbulbs coming on" be in your near future!
~DannyR~