I kind of prefer chiiruchan-san's suggestion of "Yami." Who wants to be named after money? But if you want a name that sounds similar to "Yen", you might like the Korean name "Eun." It's a female name, though-- is your new cat male or female?
Tokoro de (by the way), if the "chan" at the end of "chiiruchan" is supposed to be the "chan" which is added to a name when addressing a person, it probably should not be part of the name. One never attaches "san", "chan" or "kun" to one's own name when one says it.
Thanks for the tip on not calling chan one's own person. ^_^; I just used that because I was being called that by friends. Also I think Yami is good too... if I had a boy black cat Yami would be nice. Kuro seems overused. The name Shinkou might work too for "Midnight". Black kitties remind me of Halloween. ^_^