It's just HTML/text and it's 11TB gzipped as of 2008. I bet the archive has grown since then, but the valuable data is the oldest. Uncompressed it's probably in the 20TB range.
I'd also bet everything I own that a significant fraction of the data is unavailable anywhere else. Google Reader started in 2005. Ditto for Google Blog Search. Even if they crawled the same feeds as Bloglines and kept the data indefinitely, they'll still be missing those first two years.