Well, the blackening is a bit of a mix of many stages that they've been through. Its their most mature album musically by a long way and has the heaviness of the old stuff and expands on the melodic middle era stuff, but in a way thats not shitee.
Burn my eyes: no nonesense, young sounding but very good, heavy heavy metal. MUST HAVE
The more things change: OK, but nothing special. You wont find much like the blackening on it: there are few melodies, some solos, but not many. It maintly tries to be as heavy and aggressive as possible.
The burning red: nu-metally, some rapping, some much more melodic songs. Apparently they were under pressure to jump on the nu metal scene. You can hear some of this in the blackening (melodic riffs and clean vocals carried over), and its worth a listen, but its a very different album to the previous 2. Its not a bad album, I think. Some good songs. But this dissapointed us that were fans from the start.
Supercharger: Supergay. Only buy/DL this if you want to see how far they fell, and why they lost their record contract and why I'm so chuffed that they've risen so high again.
Through the ashes of empires: a return to form. Probably the most similar to the blackening, but not as good IMO. The songs arent as well structured or as musically intersting. If you got into them by the blackening then I reckon this should be second on your list.
Hope that helps (tis just what I think though).