久しぶりに minnty の公式を見たら,透明度関連の更新があったので。
The transparency level can now be set to any value ranging from 4 to 254 in the config file or in the command line. (For backward compatibility, values below 4 are multiplied by 16. 255 is "Glass".)
http://code.google.com/p/mintty/wiki/Changes