When 50 rock critics and DJs were asked to list the ten greatest albums of all-time in 1978, Blue didn't make it in to the compiled list (although other Joni albums did). The exercise was repeated in 1987, and Blue was no. 67 (still behind Court & Spark).
In 1997, Blue was no. 35 in the Guardian's 100 Best Albums Ever list, as voted for by panel of experts "from around the world" (although it's safe to assume most of them were British as The Stone Roses occupy the no. 5 slot).
Blue was no. 50 in the Ultimate Album List from 1999, and no. 37 in the Ultimate Album List 2 (see further down the same link).