I recently came across a 55mm f/4.5 lens that was advertised as a blue dot lens, but the blue dot is missing leaving a small recess. Is it possible that it simply fell out and/or to check the serial number to confirm that it is a blue dot lens?

Look here:
www.btinternet.com/~g.a.patterson/mfaq/m_faq-2.html

I've never seen the 55mm with the blue dot and dont think it was ever produced with one, not sure why this is.
The latest mamiya tlr lenses had stepped aperture rings with click stops at each aperture and possibly different lens coatings.
I have the 55mm its a steller lens, just by the best condtion one you can find, you'll be impressed by its results, I was.

Thanks for the link.. don't really understand what I'm looking at though. The 55mm lens has an 'FA' sticker which doesn't seem to correspond with any of the 55mm listings there.
I hear it's a great lens alright, and it seems to be in excellent condition. I had a look on google and some people have mentioned a 55mm blue dot. I guess I should have been more specific with my question though - on regular c330 lenses is there an indent on the cocking lever where the blue dot would normally be or is it only the blue dot versions have this? For e.g. if the blue dot came off an 80mm, would there be a way to cosmetically tell it apart from a non-blue dot version?

I don't think there would be a way to tell them apart. The non-blue-dot lenses have that same indent too.

My 55mm looks like a late one,hardly used, ratched aperture ring, recessed shutter cock but NO blue dot.

I think you will find that contrary to popular believe not all of the mamiya C series lenses were made with the blue dot, even the later versions, I challenge you to find a picture of a 55mm or 180mm super with one, you wont, as I believe mamiya never made those lenses with the blue dot. Quite why this is I do not know, they still have the indent you describe but never the blue dot.
The lenses I believe had the blue dot are the 65mm, 80mm, 80mm S, 105mm D/DS and the 135mm, not sure about the 250mm.
If your after the latest and sharpest lenses, get the 80mm S, 105mm DS, 180mm super with purple coating on rear element (earlier versions are yellow) and 55mm with 6 digit serial numbers. ALL these versions of these great lenses are very sharp and contrasty to match. Others may have more to add..The trick is finding them in good condition.

Perfect answer - thanks! Resident evil 5 gold edition walkthrough. The lens has 6 digits and looks to be in great condition, just need to run a roll through the camera.
Thanks for the comments people!