Maybe one of the expression gurus knows better, but I don't think it is easy for an expression to compare something like album artist across multiple albums and set a blank genre accordingly. But, every time I say something like that, somebody knows a way.
Unless you have a very large number of Album Artists, this is pretty easy to do by hand. Here are a couple of suggestions.
Since you are a smart list guru, you can just create a smartlist which contains all your albums, sort it by Album Artist and display Album Artist and Genre. You should be able to quickly scan Genre for blanks, select any of those albums with the same Album Artist and go to the tagging window and set the same genre for the selected albums. Not an expression, but pretty easy unless you have a very large number to do.
Alternatively, you could create a smartlist of just those with blank genres, sorted by album artist and scan through those, select the ones with the same album artist and change the tag manually. Since selecting the tag is a pick list, it is easy to do and you don't have to worry about typing the wrong thing.
If you have a lot of classical albums which have a tag like Composer or Orchestra you could create a smart list of non-empty Composer or Orchestra albums and tag all of those as Classical. You could do this with any tag that might be populated for a particular genre. You could do this before doing either of the first two options.
You can probably do some of these faster than figuring out an expression, unless someone knows a quite way to do this.