Hi Peter, the reason for what you're seeing is that currently ACR's Blacks control does not account for Exposure normalization. So the exact impact of a given Blacks slider value (e.g., 5) is highly dependent on how you exposed the image originally, regardless of whatever software Exposure compensation you apply in post-processing.
For example, shoot a scene in raw mode at a given exposure. Shoot the exact same scene again, but 2 stops darker (e.g., 1/4 the exposure time). Open both images in ACR with default settings, including Blacks 5. Set the Exposure of the darker one to +2 (to compensate for the 2-stop darker exposure at capture time). They won't look the same. The 2nd image will have much darker shadows (and by side effect, increased saturation and contrast). This is because Blacks=5 has a much stronger effect on the 2nd darker-exposed image, compared to the 1st much-brighter image. The opposite thing happens if you run the experiment in the other direction (i.e., 2nd image is 2 stops brighter -- you'll find that Blacks=5 has much less effect).
If you set Blacks to 0 for both images, then you'll find that they look the same (after adjusting Exposure in the 2nd image to +2, of course).
Hi Eric, I think I got it now.
Many thanks for explanation.
So in conclusion, as an ETTR shooter we should be basically not
afraid when an unusual high Blacks setting is needed in post-processing in ACR. It won’t eat the image, because the Blacks slider is then acting less "aggressive" than with a "normally", darker exposed shot. The Blacks slider is changing its effectiveness between differently camera-exposed shots, so that the setting values cannot be cross-compared. This aspect occasionally came up in the past, e.g. here
Some clarifications reached above on the initial points 1.) and 2.) about "ACR exposure / wp setting"
also allow for a more careful investigation of point 3.) plus-Blacks setting vs. black point setting via the Point Curve.
Image / screenshot 3 # 01, the darker capture (- 1 EV), was basically left again at an unaltered "linear" stage, but with an arbitrary Blacks 10 setting. Image 3 # 02, the brighter shot (+ 1 EV), was initially Exposure normalizated, then a Blacks setting of 40 was needed to match the overall brightness and to come to a numerical match with the second darkest gray. Both images then look reasonably the same, no spoiled color saturation from this strong Blacks move. With Image 3 # 03, this Blacks 40 setting was replaced by a corresponding black point setting via Point Curve, again numerically matched to the second darkest gray. This one seems to look somewhat brighter, particularly in the deeper shadows.
This result may deviate in parts from the initial example with post # 1. But then, we have been approaching more systematically since then (since post # 5). Also, it might fit better now to the insight that "in ACR, the point curve is not applied in a linear space".
Black point setting via Point Curve may possibly indicate an initial advantage regarding tonal distribution in the deep shadows, however, it may go beyond of the testing scope here if this couldn’t be reached likewise by combining Blacks with the other tonal adjustments. Same regarding black point setting via curves (i.e. the shadows slider of the parametric curve) as Andrew suggested. This aspect finally remains pretty much ETTR related, when an unusual high Blacks setting is needed in post-processing in ACR/LR, to try to replace it by Point Curve BP or Parametric Curve Shadows setting.