Interesting... I think the ability itself may need to be different (technically) than that of the singularity since I believe that singularities actually start decreasing their attack (make it more negative) occasionally.
It would be very annoying to the owner in other ways too, however, since singularities do a lot of negative things apart from just have a negative attack.
For instance, they cast a nova for your opponent sometimes.
Properly adjusted, however, this would be a very amusing creature to watch.