Macbook display black on boot up with external monitor
OS X remembers the configuration for each individual external device you plug in and should restore the last setup.

You should get an extra icon in the top menu bar when you have an external display plugged in, and clicking on this will give you the options to rearrange the displays and choose which are on. 

It may be that you are booting up into 'closed screen' mode (see this Apple support article).

You don't say which version of OS X you are using. Before Lion the behaviours were different and if so things might improve after upgrading.