The definition I found for the blueprint css class called .column says:
.column Use with .span-x classes to create combinations of columns in the layout.
But the span-x classes seem to overlap this functionality since they allow columns to be defined without referring to .column?
Could someone explain what distinguishes these two classes?
the .column class has been dropped. it used to be used with .span-x to define a column but the css was updated so that a div with a .span-x defined a column by itself.
see line 100 at http://github.com/joshuaclayton/blueprint-css/blob/e9e91bb94a0f498e63359556b2301cfd18400377/blueprint/screen.css
the column class is the same as any div with a span-x
take a look at http://www.blueprintcss.org/ for the latest version.
Josh