mounting raw disk image - xgbn/handbook GitHub Wiki
hdiutil was the right way to go but a crucial ingredient are the parameters.
The command
hdiutil attach -imagekey diskimage-class=CRawDiskImage -nomount filename
perfectly mounts the block file as a block file system. Afterwards you can mount any fs like Chris S pointed out.
http://serverfault.com/questions/174909/mount-block-file-on-osx