The BLAST package for MacOSX is under the same directory as for Windows and UNIX platform. It is named with the following convention, with universal-macosx indicating the platform, e.g. PowerPC or Intel chip based Mac with OSX: blast-#.#.#-universal-macosx.tar.gz.
BLAST installation under Macintosh is essentially the same as under other UNIX systems if we choose to download it using ftp client and performs the installation using command line. If we choose to download it using browser, StuffIt Expander should automatically expand the archive to install the blast-2.2.15 folder (for current release) on the desktop. We should create a db folder under it and move the blast-2.2.15 folder to the home directory, which generally resides at: HD icon, Home icon, icon with your name.