Unseen companion to Sol

I read in ScienceDaily the other day that the orbit of one of the
newly discovered minor planets indicates that the sun is part of a
binary system. The problem I see with this is that we would have seen
it by now. It can't be Alpha Centauri, it's too far away. There are no
closer stars that could be companions to Sol. If it was a brown dwarf,
it would be big enough that we would have seen it by now, wouldn't we?

So here's my idea, it's a black hole. One that has "eaten" all the
matter around it and is silent. It's not emitting anything because
it's not taking anything in. The only way to "see" it would be to
watch the entire sky until a star unexpectedly is "magnified" by the
gravitational lensing when it passed by. Since neutron stars are less
than 10 miles across, how big is the event horizon on a black hole? A
mile? Half that?

I have gotten the impression that a black hole "sucks" things into it.
On tv they show them taking in entire solar systems and such. But it
seems to me that the black hole wouldn't have any more gravity than
the star that made it had to begin with. It would just be that the
star is compacted into a tiny spot. While I figure that would disrupt
the orbits of any planets around the star and cause them to either go
off into space or into the black hole. But after that has occured, it
should go dead. Shouldn't it?

Well, is it a theory?