I don't know whether or not the blue dot is it, but the original poster is 100% correct about needing a second sensor to determine the actual aperture of the lens.
The Contax G1 and G2 use a very similar system. The external meter cell was provided primarily for exposure setting when using the 16mm Hologon lens (which sits too deep for TTL metering to work) but also is used to derive the aperture setting in use. The only use made of this data in the G cameras was to imprint the exposure data on the G2's data back, but obviously with a digital camera more can be done with it.
Here's the background of why an external sensor is needed to determine the f/stop in use:
All SLRs that feature full-aperture metering communicate the aperture setting from the lens to the metering system in the camera body, either by a mechanical linkage (e.g. the prong on Nikon F lenses) or an electronic contact (as used on most modern SLRs.) This coupling reads the actual setting of the lens' aperture ring and passes it along to the metering system in the camera body.
The metering system needs to know the aperture setting so it can determine the correct shutter speed without having to stop down the lens, which on an SLR would cause the viewfinder to get dim. (Owners of stop-down-metering SLRs such as Pentax Spotmatics are all too familiar with this.)
Rangefinder camera lenses (including Contax G-mount lenses) have never had this kind of aperture coupling system. They always meter at the actual working aperture, with the lens stopped down. It's a simpler and theoretically more accurate system, and there's no reason not to use it, since you're not viewing through the lens and don't have to worry about the finder getting dim when you stop down.
But because the aperture ring setting isn't communicated to the metering system, the meter doesn't know (or care) what aperture you've set. It just measures the amount of light passing through the lens, and determines what shutter speed is appropriate for that amount of light. It has no way of knowing whether you're metering a dimly-lit scene at f/2 or a brightly-lit scene at f/16... and it doesn't need to know that to give you a correct exposure.
However, if you're trying to record your exposure data (on a data back or in an EXIF file header) you'd naturally like to know the aperture to which you had set the lens. So, Contax came up with a clever trick which Leica apparently is adopting as well: Measure the brightness of the
scene using a NON-through-lens meter, and compare that to the brightness read by the through-lens meter. By comparing the two, the camera can compute what lens aperture would be needed to account for the difference in brightness. The result will only be approximate, but it should be close enough to help you evaluate your exposures.
[At first I thought it also would be necessary to know the lens' maximum aperture, as it is on SLR full-aperture metering systems -- but further thought made me realize this isn't actually necessary, so I've deleted that part of this post.]
To give a simple example I thought up for another mysterious-blue-dot thread, suppose you see me taking a picture and ask what settings I'm using. I reply, "My ISO is 400 and my shutter speed is 1/125, but I'm not going to tell you the aperture." (I must be crabby that day.) Still, if you happen to have a hand-held meter with you, you can set it to ISO 400, meter the scene, and then see what aperture lines up against 1/125 on the calculator dial, and you know approximately what f/stop I would have been using. The Contax G2 data back and presumably the Mysterious Blue Dot work exactly the same way.
As to the question of automatic enhancements, I can think of one example right off: vignetting. Most of us have observed that vignetting is worst at full aperture, and improves as you stop down. If you're Leica and you want to include automatic vignetting compensation, you'll need to know the lens' working aperture as well as its focal length, so you can apply an appropriate amount of correction for the aperture in use.
It will be interesting to see how they use this data: whether the camera includes a lookup table of correction values derived for various Leica lenses and applies correction to the images "on the fly" (which might mean that third-party encoded lenses could give inaccurate corrections) or whether they just store the coding data in the file header and then use it for post-processing via raw conversion software.
Picking apart this kind of stuff will be lots of fun for technically-minded RF enthusiasts, even if we can't afford to buy an M8 ourselves!
