"Aforar" can be used intransitively, as defined in the DRAE, or it can be transitive, as in your example: "aforan la parrilla", etc.

"8. intr. Dicho de una decoración teatral: Cubrir perfectamente los lados o partes del escenario que deben ocultarse al público. U. t. c. tr."
http://lema.rae.es/drae/?val=delirante

In English we don't really have an intransitive verb for this. If you needed to translate "aforar" used intransitively you'd have to say "mask the backstage areas", or something like that. Material used to conceal the backstage area from the audience is called masking, and "mask" can be used transitively, as in your example:

"Masking: Basically this is scenery or other visible material designed to hide backstage stuff the audience is not supposed to see, such as the wings or the back wall."
http://www.lagunaplayhouse.com/downloads/glossaryoftheatrete...

"BLACKS
2) Any black drapes or tabs, permanently or temporarily rigged. Used for masking technical areas.
[...]
BORDER
A narrow horizontal masking piece (flattage or cloth), normally of neutral colour (black) to mask the lighting rig and flown scenery from the audience, and to provide an upper limit to the scene. Often used in conjunction with LEGS."
[...]
Masking flats are used to hide areas the designer does not want the audience to see"
http://www.theatrecrafts.com/glossary/results.php?searchtype...


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Note added at 54 mins (2014-01-03 23:16:08 GMT)
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"MASK - To obscure from the audience view.
MASKING - (or MASK) Common name for a flat, drape or groundrow or set piece used to mask area of the stage from the audience."
http://barbershop.org/document-center/category/13-performanc...