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.
While the original post inquires specifically about gravitational lensing, I'd like to suggest that this is not the only criteria for "seeing" a black hole. Clearly, since no light can escape from a black hole, the actual hole itself cannot be seen. However, there are a variety of epiphenomena that can be observed, and gravitational influence on other celestial bodies (e.g., stars) can certainly be observed.
The European Southern Observatory (ESO) released a video of the center of the Milky Way galaxy made from 20 years of observations of this area by the NACO instrument on ESO's Very Large Telescope in Chile. This seems to me like extremely compelling evidence that an enormous black hole exists at the center of the Milky Way.
As for gravitational lensing, Space.com reported just such an image attributed to Courbin, Djorgovski, & Meylan. This image may satisfy the exact criteria requested by the OP.
EDIT/UPDATE: This morning (April 10, 2019), the Event Horizons Telescope held press releases on four continents announcing that they have indeed imaged M87*, a black hole of an estimated 6.5 billion solar masses in the Messier 87 galaxy, some 55 million light years away. Strictly speaking, we still cannot see the black hole itself but rather its epiphenomena such as the accretion disk and the turbulence caused by the presence of the black hole. This image, created by very long baseline radio interferometry, is not a photograph in the traditional sense, but rather a false-color image composite generated by converting enormous volumes of radio wave observations with a supercomputer into an image visible to the human eye.