You can do it with the physics behavior. Turn off gravity with the action "set world gravity to 0" and use "apply force toward position". The position is the black hole and the force would be something like k*1/distance(object.x, object.y, blackhole.x, blackhole.y). "k" is basically how strong the black hole is. Change "k" it to whatever value you like, a higher value should give good results.
The objects will just move toward the center of the black hole unless you give the objects some velocity 90 degrees from the direction to the blackhole.
Here is a further discription with some useful diagrams:http://www.physicsclassroom.com/class/circles/u6l4b.cfm
And here is where I got the gravity formula:http://en.wikipedia.org/wiki/Newton%27s_law_of_universal_gravitation