Blacks have a stiffer feel to them and are linear, meaning there's no tactile resistance (ie a bump or something once it's actuated). Blues have a lighter key and click when actuated, in addition to having tactile feedback.
According to the link I provided (I don't have any actual experience with these, so that's my only source), Browns should have a somewhat lighter tactile feedback than Blues, without the click of course.
Check out the "Key Feel" section of this page for more info on Browns: http://deskthority.net/wiki/Cherry_MX_Brown