I am also seeing this issue. I have reported it to stardock already. Anyways I have found it to occur on any skin. I have tested on the following skins.
Diamond (included with WindowBlinds)
Aero Metal (included with WindowBlinds)
Gridlok (listed as a Master Skin on Wincustomize)
Here are some example screenshots I have made real quick.
With WindowBlinds applied:
With only the Windows XP Theme:
Oh and since you ask for it here is my system info.
WindowBlinds version : WindowBlinds 6.3 (build 68 x86)
WindowBlinds is installed correctly on this PC
WindowBlinds appears to be activated on this PC
Your machine supports per pixel borders on WindowBlinds skins.
(Default Monitor) 1 is attached to NVIDIA GeForce Go 7900 GS
Wblind.dll 2008/10/18 15:42:20
Wbsrv.dll 2008/09/16 07:44:42
Wbconfig.exe 2008/09/16 07:44:32
Wbload.exe 2008/04/26 15:14:14
Wbhelp.dll 2008/04/26 15:14:07
Wbui.dll 2008/04/26 15:14:05
Tray.dll 2008/04/26 15:14:21
Screen.exe 2008/07/15 10:31:09