Well before I can answer your question I need to know what version of Tamagotchi you have, It says it on the package or if you threw if out then maybe you can search it to find out.
If you have a Tamagotchi Music Star, then to get a Kuromametchi you need to try to take care of it perfectly. Play games regularly, dont wake it up while sleeping, and make sure to keep its happy and hungry hearts up.