Exporting Blocks from Drupal 6 to 7
Assuming they are straight up Blocks with body content (not View blocks or other contrib generated blocks), I would create a module that scripts and creates them on install. If you have many blocks, do them in small batches (breaking them up across update_n hooks).

That way you can also test them in small batches and get it right. Once up, you can disable the module and remove it.

See:


https://drupal.stackexchange.com/a/72946/57
https://www.drupal.org/node/1709954#comment-8592077