This is a good observation, but gravitational lensing surveys have all but eliminated massive compact objects as a potential source of dark matter; see for example https://www.ncbi.nlm.nih.gov/pubmed/17359015.
The NIH (specifically PubMed) just indexes most scientific journals. This paper is actually in Phys. Rev. Letters (which the NIH site links to, if you click the DOI link below the abstract):
Gravitational lensing surveys coverage is pretty darn small so far it's also might not be appearant on the scales we are talking about there will need to be a pretty lucky alignment of a galaxy and a black hole given the space and relative size of both objects and the noticeable lense diameter I'm not sure that intergalactic lensing will be detectable as intragalactic ones which also are pretty rare.