It seems I can't use IDEA under Blackdown Linux JDK. Blackdown 1.4.1 is based on Sun 1.4.1_01, but the internal version number is just 1.4.1, so IDEA complains about that, saying it needs 1.4.1_01 or higher.
I think there should be a way to force IDEA to run on any JDK the user wishes (ie, just remove the check and let the program run at the user's risk).
Blackdown Linux JDK offers huge improvements on Java2D performance over Sun JDK, so IDEA should run faster under it. Please consider that :)