They say that the system can be seen with the unaided eye. If I were to point my telescope directly at the black hole, what would I see? Nothing? Or a lensing effect of the stars behind it?
Also, using "your telescope", you won't see the orbits of the black hole's companions with sufficient precision to deduce that the black hole exists and that it's a black hole.
I'm curious why there's no lensing, though. Or is it just that the size makes the lensing weak enough to not be noticable?
That's why gravitational lensing has only been observed for
1. very bright stars behind the Sun (event that lasts a second detectable with sub-second exposure)
2. Galaxies behind galaxies (event that lasts millions of years detectable with hours-long exposure)
3. an SMBH warping the light generated by its own disk (permanent event detectable with years-long exposure)
This phenomenon is known as microlensing [1] and it's one of the ways that exoplanets have been discovered because the exoplanets also lens the background star and produce a characteristic spike on top of the light curve.
That said, microlensing is sort of statistical by nature. It's very unlikely that any particular object will be aligned with a background object just right to produce noticeable microlensing. But with a large field of stars and enough observations it becomes possible to observe a microlensing event every now and again.
[1]: https://en.wikipedia.org/wiki/Gravitational_microlensing