[Tloewald]

Also even small (stellar) black holes are very massive — 5x sol and up — (since they tend to occur from the collapse of very large stars) so having one pass near or through the solar system could be catastrophic.

Primordial black holes are posited to be possibly so small as to be able to pass through the earth without interacting with more than a few atoms, which would make visiting them both difficult and not enormously interesting.

Edit: clarification and added info on primordial black holes which I looked up.