So was the point of your thread to say that Kamui can be cancelled out by Shinra Tensei or not? Because that's what your title implies, but it seems like your explanation proves you wrong, unless I'm missing something (and I could very well have).
Shinra Tensei pushes out. Kamui pulls in. The difference between your example and what would actually happen is that blackholes typically both pull mass in, which creates the tension between them (gravitational pull) that will either make one cancel the other out or both of them cancel out.
It's just like a game of tug of war. Let's say two people grab each other's hands and start pulling. If the person on the left is stronger than the person on the right, then the person on the right gets pulled in and vice versa. If they are of equal strength, then either they will both remain in the same place or they will lose grip, thus cancelling out both players by disrupting their pulling ability. The key here though is, they are both pulling.
But if the person on the left is pulling (Kamui) and the person on the right is pushing(Shinra Tensei) then the person on the left wouldn't be cancelled out or pulled in at all. The person on the right's pull would be cancelled out and he would be sucked in since he is pushing his force toward the center of gravity to the person on the left, in other words, he is actually helping the person on the left pull.
Kamui pulls towards it's own center of gravity. Shinra Tensei pushes away from it's own center, and would push outward toward Kamui if they were both used at the same time. This does nothing to disrupt Kamui's pull, it actually helps it. Think of it this way, If Shinra Tensei is pushing someone away, while Kamui is trying to suck them in at the same time, what would actually happen is Kamui would suck them in faster than normal, since it has the added force from ST helping it.
I think the jutsu you meant to say was probably Bansho Ten'in, which is just like Shinra Tensei except it pulls in. Bansho Ten'in could possibly cancel Kamui if it pulls hard enough, but Shinra Tensei could never cancel out Kamui