Welcome to WebmasterWorld Guest from 188.8.131.52
Welcome to WebmasterWorld [webmasterworld.com]!
Here's a good start: Introduction to mod_rewrite [webmasterworld.com]
How you do this rewrite depends on what you want the URLs used by users and SEs to look.
Do you want the subdirectory "blah" to show in the address bar, or do you want its existence to be transparent to the user?