Yeah, I really hadn't decided on the shadow color so I left it for now. Actually, it isn't really black but just a touch off of black (R=8 G=8 B=8) so I wouldn't have any black details semi transparent. Now that you mention it I would like to see what the RTP shadow color looks like on here. Thanks for the feedback!
EDIT: The RTP shadow color didn't quite look right, so I tried the shadow color from a Breeze tileset and it looks a bit better. Here's a screenshot: