Zero density, although that is a bit of a misleading answer.
The Kugeblitz (1) is a black hole made out of radiation, like light. This is because mass and energy are interchangeable, so having enough energy in a small area means you can create a black hole as well. In this case by concentrating so much light through a single spot at a single moment that it collapses. This might also be the only way to feed a black-hole powered ship since it is exceedingly hard to feed a subatomic black hole with ordinary matter consisting out of atoms.
This does prove one thing: the mass of the object does not matter, what matters (heh) is getting enough density or energy in a single point of space, which will then form a black hole.
Just be sure to check that calculator from L.Dutch's answer. It also mentions something else: the lifespan of the black hole. Any black hole will evaporate (although this is actually an unconfirmed theory, but our best guess at how one works). This evaporation causes energy to be released, and the smaller a black hole is the faster than evaporation process. This process converts mass to energy, and the last seconds of a black hole surpass nuclear bomb levels of energy release. A black hole that lasts a day weighs 1.22928E7 kg for example. Your taco wont be close.