Depends on the size, position, and number of black holes, right? We see lensing currently because of super massive black holes that we know about. But if there's a bunch that are basically as massive as our sun (or less) then we are dealing with event horizons ~3km or less. It'd be pretty hard to spot those as the diffraction would be rounding errors.
My logical leap is that if blackholes constitute our measure of dark matter (25% of proportion) or dark energy (68% of proportion) that they'd attract each other and glob together. Also true of gravstars.[0]
