I'm not sure what you mean by "alias", but a symbolic link is probably what you need. Close Bitcoin-Qt and Armory, move the blockchain folders wherever you want them, then run:
ln -s <destdir> <sourcedir>
<destdir> is the directory you moved the files too, and <sourcedir> is the directory where they used to be and in which Bitcoin-Qt expects them (should be ~/Library/Application Support/Bitcoin).
To regain the most space, you should link Bitcoin-Qt's "blocks" and "chainstate" directories, as the remaining files are miniscule and some are sensitive (wallet.dat). I don't know the specifics for Armory, but it's most likely very similar.