This only happens when the partitions are not already mounted when you start gparted.
The workaround is to:
1) Mount all partitions on the drive or device
2) Start gparted and allow it to scan the device
3) Then unmount the partitions.
This prevents the icons from appearing in the Launcher and keeps the partitions in the blacklist, although in most cases its more trouble than simply removing the icons after they appear.
I browsed the code for both Gparted and Unity and here's a very simplified summary of what is happening.
1) Gparted scans the system and builds a map object listing partitions.
2) Unity detects this and sees the unmounted volumes in the map as newly added volumes, even though they are already listed in /proc/partitions and other places.
3) When Unity detects a new volume, it creates an icon for it and adds it to the Launcher. Part of the process of adding the icon is to remove the volume from the blacklist.
A closely related issue is that Unity removes removable devices from the blacklist on eject. If you blacklist a removable device from the Launcher, it will always come back.
I can understand how this behavior may be needed for phone and tablet interfaces, but I see the bug as a need for a little more analysis of these features. The problem for many power-users on the desktop is that logical partitions are seen by Unity as removable devices. If you plug in a usb hdd with, say, two primary partitions a dozen logical ones, you can blacklist the primary partitions but the logical partitions will always show up on the launcher with a usb device icon.