Have we seen a black hole? Have we actually seen a black hole out there? By "seen" I mean "recorded a em-spectrum image of gravitational lensing caused by object believed to be black hole".
The answer was no prior to 10 April 2019. There were no resolved images at any wavelength of black holes or black hole candidates that demonstrated their lensing effect.

There were of course lensing images due to massive objects that probably have black holes at their centres (e.g. Courbin et al. 2010 and see below), but that is not the same thing.



A quasar acting as a gravitational lens - Courbin et al. (2010)

All this has changed with the release of a reconstructed millimetre-wave image of the supermassive black hole at the centre of M87.

The image below, obtained by the "Event Horizon Telescope" (a global interferometer) is formed by intense light from accreting matter being lensed by a 6.5 billion solar mass black hole. The dark circle is known as the "photon shadow" and is about two times the size of the black hole event horizon. The bright ring is caused by the lensing of light from around and behind the black hole. It is thought to be asymmetric and off-centre because of the black hole rotation.