Kurodake's opening and closing dates for the season are up on their website. It's already open (with about a metre and a half under the chairlift).
The whole place closes down from January 4th to 28th. The gondola re-opens on the 29th of Jan, but the chairlift will still be closed until the the 18th of Feb. You can still lap the gondola while the chairlift is closed, but you'll have to pay ¥1,100 per ride.
All day tickets (¥3,800) and good times resume on the 19th of Feb. See you then!