[jules]

So you could say that all point masses (e.g. electrons) are black holes?

What happens if you take a large black hole, and you throw a lot of electrons into it? Does it get an electric field measurable from the outside? If not, how come an electron does have this field?

[lutorm]

An electron is only a point mass in classical electrodynamics, which leads to all kinds of inconsistencies (the self-energy of its field would be infinite, for example.) In quantum mechanics, there is no such thing as a point mass.

But in any case, the answer to your question is yes. Black holes have 3 quantities: mass, angular momentum, and charge. So, yeah, you can charge up a black hole by dropping charges into it and the charge would be visible to the outside.