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This can be dust. If it is dust and the edges have some definition, then it will be AFTER the optical assembly, between the assembly and the lens. Otherwise, it would be some between the lamp (not ON it) and the mirrors. The reason I suggest this is that you mention that the spot is black - if it is something hardware related, it will more likely be a color other than black. It's still not impossible, but I would check this first.
The lens is in the front, yes. And the lamp is in the back. What I would suggest doing is, without disassembling it, is to blow it out with compressed air. You are best not to open it if you can avoid it. Some of these are supposed to be sealed systems... though they don't necessarily remain that way forever.