"Blue" introduces us to the Blue Kryptonite that strips Clark of his powers. I liked Zor-El's plan to get the ring on Clark. It showed Clark's human weaknesses and how they'll always be a constant threat to him as he evolves into Superman.
More importantly than this, the episode also reveals that Grant Gabriel is actually Julian Luthor, which is a revelation that I absolutely loved.
Kara also gets sent to Detroit with a free memory wipe to boot, which is nice because... she really doesn't have much to do other than look hot, which she does incredibly well.
"Blue" holds up as one of the better entries in the pre-strike run and is worth a watch.