Stereochemistry in Sn2 reaction I'm confused why the absolute configuration always has to change after an $S_N2$ reaction:


For example, in primary alkyl halide above there are 4 planes Nu can attack; and absolute configuration is changed after an $S_N2$ reaction because the nucleophile (Nu) only attacks the 'black plane'.
I can understand that the nucleophile wouldn't prefer red or green plane because of steric hindrance, but here's my question:

Why doesn't the nucleophile attack the 'blue plane'?
The attacking nucleophile does not just shove its electron pair to the carbon from some random direction. No, it must donate that pair to the antibonding orbital ($\sigma^*$) of the $\ce{C-X}$ bond, thus weakening and eventually breaking that bond. And it just so happens that the antibonding orbital is mostly located on the side opposite to X.

Upd. Thought I'd better look up a nice picture. Here's one:

(from this page)