  We discuss a "compact source" model of very high energy (VHE) emission from
blazars in which the variability time is determined by the blazar central
engine. In this model electron or proton acceleration close to the supermassive
black hole is followed by the development of electromagnetic cascade in a
radiatively inefficient accretion flow. Assuming such a model for the TeV
blazar PKS 2155-304, we show that the variability properties of the TeV
gamma-ray signal observed during a bright flare from this source, such as the
minimal variability time scale and the recurrence period of the sub-flares,
constrain the mass and the angular momentum of the supermassive black hole.
