Hrm. Even turning off "cast shadows" didn't do it. It's basically not a shadow...it's that skylight (and, seemingly, indirect light originating from the sky dome and bouncing off a bounce sheet), will not pass through transparency. It could be it will happen if there is enough space between transparency and object (I recall trouble with Poser eyes before on this account). Alpha channel won't fix it any more than any other materials setting. As a double-check I disabled skylight and tried a couple ordinary lights. All of these operated normally, and didn't create dark blotches below the transparency. This is very annoying. It basically blocks me from using the most attractive lighting options in Carrara on imported Poser figures.