found a solution by myself.
For anyone stumbling over this question:
Motion blur uses custom depth and is drawn in front of the rest of the scene, so translucency has no chance to get involved.
Used a workaround to tint the blur itself accordingly:
added a color modifier to the blur material, see picture, created two instances, and adjusted one so that the color of blurred object matches the translucency tint.
Then switch them dynamically depending on slide door down or up.
