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Just to be sure since you can't use those two controls in LR at the same time:
A. Are you asking about the difference between "fill light" in LR3 (old 2010 develop process) and "Shadows" in the LR4 (new 2012 develop process)
B. Do you really mean the difference between "Fill light" and "Blacks" in LR3/old PS 2010 develop process?
If "A", then the difference is that Shadows in LR4, while maybe not better for every image, is generally an improvement over LR3. Tweaks were made to the tonal range impacted and also the shadow/edge detection algorithms used (to reduce halos). Shadow tones can now be darkened as well as lightened with the new slider. The amount of Shadows you use in LR4 will likely be different than what you are used to with Fill Light in LR3. I would say that the changes/improvements made to Blacks and Shadows is more subtle than the changes Adobe made to the highlight end of the tonal range with the Highlights and Whites sliders, but I do believe there is a noticeable improvement.
If "B", Fill Light impacts all shadow tones. Blacks sets the black clipping point. They interact a bit but there is a definite difference in what they do.