Can I ignore the blocks directory for time machine backups on OSX? After installing Bitcoin-QT I have over 40GB of files in the ~/Library/Application Support/Bitcoin. It seems that the most important file there is wallet.dat. Does it mean that I can exclude from time machine backups the ~/Library/Application Support/Bitcoin/blocks subdirectory to reduce the backup footprint? Will the bitcoin client download again those files in case of backup restoration without this subdirectory?
Yes, it is completely safe to exclude the blocks directory from your backup process.

In the event that you lose all your files and need to restore from backup, Bitcoin will download all missing blocks. This may take some time, but there is no information in that directory that is unique to you.