"According to Newton's law the negative mass should be repelled" -- Nope, in both Newtonian physics and in general relativity, negative mass would be attracted gravitationally to positive mass, although negative mass would exert a repulsive gravitational effect on positive mass (but if the negative mass is small compared to the mass of the black hole this latter effect is negligible). In Newtonian physics this is not too difficult to derive, the Newtonian gravitational force law indicates the gravitational force vectors between a positive and negative mass would point away from each other, so the positive mass is obviously repelled, but for the negative mass the acceleration is in the opposite direction of the force due to the negative mass in F=ma, so the negative mass is attracted. In general relativity the analysis is obviously more complicated, but Hermann Bondi showed negative mass would have the same basic properties in GR, see this article. Note that if negative mass didn't fall downwards in gravity just like positive mass this would be a violation of the equivalence principle, since being in a chamber at rest in a gravitational field is supposed to be equivalent to being in a chamber accelerating in deep space, and if you let go of both a positive and negative mass in such a chamber they should naturally just move inertially while the floor of the chamber accelerates up to meet them.
The situation of negative mass falling into a black hole does have one important consequence though, in GR it's the only way for the event horizon of a black hole to shrink rather than expand, and for this reason a dynamical black hole metric (the Vaidya metric) with negative mass falling into it is sometimes used when trying to model the long-term behavior of a black hole that is "evaporating" due to continually emitting Hawking radiation (since this is a quantum effect, and general relativity is not fully compatible with quantum mechanics, this evaporation should ultimately require a full theory of quantum gravity to model it completely accurately, but it seems reasonable to expect that the earlier stages of evaporation, before the size of the black hole and the energy density approach the Planck scale where quantum gravity effects are expected to become significant, should have some close analogue in classical general relativity). See for example the paper here, whose abstract says "the black hole evaporation due to the Hawking radiation that is modeled by the Vaidya metric with a negative mass", or section IV of this paper which uses the Vaidya spacetime to model a black hole and says on p. 4 " This matter energy is negative near the event horizon. In the dynamical horizon equation, if black hole absorbs negative energy, black hole radius decreases. This is one of the motivations to use the negative energy tensor."
