I checked the Gibbons catalogue but there was no mention of an error like that for that issue or the base stamp itself (i.e., without the overprint), I looked at my sets (mint and used, I do like Mauritius) and none of them had "the queen's birthmark" (or whatever you'd like to call it).
Can you upload a sharper photo or scan? The picture is too blurry to really see it well.
My gut instinct is that it's a bit of schmutz from something other than the printing process. Could be dirt, perhaps ink...? A clearer scan will be really helpful. :)