Is it the lens or is it the camera producing a black blob on every photo? How can I tell which is at fault? It is a used Nikon camera kit I have just purchased and there is only one lens so I can't do a comparison test.
i suspect you have a dust spec on your digital sensor (assuming here that this is a DSLR).
Fit a lens and set the aperture to be small ~f16 and take a photo of a light surface, you should see your blob. Change lens and repeat.
If the blob is still there then you have a dirty sensor that needs to be cleaned. If is not then you have a dirty lens and that needs to be cleaned.
 Since it is a recent purchase, is it new or s/h ? either way the most likely culprit will be sensor dust bunnies - can you go back to the retailer ? [edit over]
Especially if your camera is old, the most likely candidate is that your sensor is dirty.
You can find this out, if you
- put your camera in A-Mode (on the dial).
- Then turn the wheel so that Aperture is f/16.
- Face a white wall.
- In manual focus, focus very far away (infinity). This way you won't see the structure of the wall.
- Take a picture.
- Inspect the picture for specs of dirt (it might be easiest if you do this on a computer).
If you can see the blob clearly with this test, then the blob is on your sensor. Look on this site for questions on how to clean your sensor, or have it done in a camera shop.