When you put your ColorChecker in the shadow then you mustn't wonder why the sun-lit areas will come out too warm.
Blue sky shade is very "hot" color temperature wise, as you've discovered. It's the traditional bride's white dress problem, move her to the shade and all looks ok to the eye but the camera will record her white dress as light blue. If you white balance off of the color checker neutral patches in the shade, then the sunlit portions will appear yellow. If you white balance off of the neutral patches in direct sunlight, then objects in the shade will have a blue tint.
IMO blue in the shade appears more natural than overly yellow sunlit objects, unless the shade object is somebody's face. The best "solution" is to place the subject in reasonable shade without a lot of bright sunlit areas in the image. Or get up early and use that beautiful morning light.
Thanks for the quick responses!
I understand that shade tends to appear blue, while sunlit areas appear. That makes total sense to me.
However in most of my shots absolutely everything is yellow, even the shade!
I found some better examples that also illustrate how holding the chart at a different angle will reduce the white balance temperature from ~8000K to ~6000K:
As you can see, in the first picture, the chart points nearly straight up, but is in the shade, just like my subject.
In the second picture, the chart is more angled - which I guess makes it receive more bounced sunlight - now the shade is slightly blue and the lit areas rather neutral.
It kind of makes sense - I'm just wondering if my assumption is correct, that pointing the chart upwards will make it receive that much more skylight, turning it way blue.
The ideal chart angle is 45 degrees to the illuminant, and head on to the camera. However, it shouldn't be very sensitive, as long as the illuminant is not head on. I suspect in the shot where you are handholding the color checker the light striking it is a combination of blue sky and reflection off of whatever you're wearing. And setting it down on the ground may well pick up a lot of green light from the grass.
Lighting 101, you've got to check out your environment carefully. Best bet is to have somebody hold it out in front of them, off of the ground and only receiving the light that eventually will illuminate your subject. Or prop it up somehow, get it away from reflective surfaces.
Added by edit - the shot with the color checker up against the brown box will certainly "pollute" the light striking the cc.
I downloaded your image where you are hand-holding the cc, assigned sRGB to it, and poked around measuring rgb values. The neutral patches on the cc measure very close to neutral, i.e. R=G=B. However, the shady part of your socks are way off neutral, so for whatever reason you did not have consistent illumination on the entire scene. Again, I suspect reflection from your clothing or something similar, biasing the cc illuminant.
One of the guys who runs another site has developed a thing that's basically a diffusing filter (I think it's called a ColorRight or something very similar) which measures the light reaching the front of the camera.
It's not too terribly different than an "auto white balance" in the camera, except that it takes into account a wider field of view than what reaches the sensor in general. I won one in a photo contest, and it does work as advertised. Surprisingly well in some situations.
Since it's not measuring the illuminant (of which you have multiples of varying temperatures - sun, sky, reflected light) but rather the lighting of the overall scene, maybe it could help you with tricky lighting situations such as what you're describing here.
> whitebalance all shots from the same location, using the second brightest patch on the macbeth chart
actually, formally, you should use a dedicated WB patch for that... patches are on the chart are not that neutral = http://www.rmimaging.com/information/ColorChecker_Passport_Technical_Report.pdf