Ideally, smaller events (not the big rock show, usually) where the musicians are fairly close together. Blumlein's 45 degree angles can sometimes introduce phase issues when the soundstage exceeds the 45 degrees left and right. A jazz environment lends itself nicely to the use of a blumlein array (trio, quartet, etc). In some situations placing the mics directly in the center of the musicians yields amazing results.
It doesn't work particularly well from distance, due to the over-reverberation that can sometimes occur. Just think about what you're picking up in a blumlein pair, and you'll understand why something recorded from "the section" probably won't do quite so well. It's possible, but not recommended.
I ran it last night at a small club rock show, about 10 feet from the stage and about 10 feet up. What I managed to capture sounded very nice to my ears.
It may be important to note here that BLUMLEIN and M/S are not the same thing. M/S involves the use of a directional mic (cardioid is traditional, although omnis and sometimes even hypers can be used) as the MID, and a figure of 8 capsule as the side. Blumlein, named after a technique devised by Alan D. Blumlein, involves two figure of 8 capsules mounted head to head (so as to occupy as close to the same space as possible) with one capsule angled 45 degrees left and one capsule angled 45 degrees right. So your top mic stores information from the left front and right rear lobes of the pattern, and the bottom mic gets the right front and left rear lobes.