Awesome work! Something I wonder, would an observer actually see a black circle? If the photons are fanned out when they pass near the black hole, wouldn't some of those photons serve to obscure the black hole itself?
I don't think the gravitational lensing would obscure the event horizon, but if there were objects far enough in front, those would. We see things based on where it appears light has come from if it were traveling in a straight line, not just on where it hits our retinas. You can think of lenses as changing the direction of where light goes, but you can also thing of them as changing the apparent direction of where light has come from.
In order for light from the gravitational lensing to obscure the object causing the lensing, it would need to appear to be traveling in a straight line away from it. I don't think this is possible, I drew up a little sketch to try and illustrate this: https://sketch.io/render/sketch552c20b7b0f9f.png.
If the object itself is emitting light, then you will see it, but if it is a black hole, it would appear to a black circle
