[acecharly (OP)]

Is it possible that a blackhole or some object could have so much mass and was so huge we would not realise it was a gravitational lense?

[lean bean]

Is it possible that a blackhole or some object could have so much mass and was so huge we would not realise it was a gravitational lense?

Wouldn't you expect a large enough mass to deflect light?  And if the alinement of source, mass(black hole) and observer is right, wouldn't you expect it to be produce an Einstein ring. In other words, why would you expect a large enough mass not to be a gravitational lense if the alinement is right?

[evan_au]

Gravitational lensing of distant galaxies is thought to be due to objects much larger than a single black hole - perhaps a cluster of galaxies. See https://en.wikipedia.org/wiki/Gravitational_lens

These generally distort the image in space to such an extent that with some study, you would detect that it is gravitational lensing - sometimes because it produces several distinct images of the same distant object.

It is also possible that much smaller objects within our galaxy can cause microlensing which is detectable with current technology, including individual black holes, neutron stars, and "brown dwarf" stars too small to initiate fusion (ie free-floating oversized Jupiters). These distort the image brightness and shape, and vary over time as the Earth, the distant object, and the intermediate object move relative to each other. So this would be fairly easily recognisable as gravitational lensing.
http://en.wikipedia.org/wiki/Gravitational_microlensing