Attached are screenshot of the black outline of blocks when the cursor moves over them in versions 1.7 and then 1.7.1. Notice that the black outline is much darker in version 1.7.1.
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I dont think this is a bug, I think they did this because the outline was invisible when you tried looking at blocks through water or something like that. I like the darker bounding box.

It is very annoying IMO

Like the old, more unvisible one more..

It's a bug. I remember this happening in one of the 1.5 updates and they did fix it.