Situation You washed your hands, said ha'motzi, were eating your meal (this could be se'uda shlishit or even a fourth meal) and it is now dark. You want to do melacha.

What To Do If you have not yet finished se'uda shlishit, saying Baruch ha'mavdil bein kodesh l'chol after dark on Saturday night does not affect the Shabbat additions you will then say in birkat ha'mazon.  So, you may say Baruch ha'mavdil bein kodesh l'chol and do melacha, and then continue to eat your meal or say birkat ha'mazon INCLUDING the two Shabbat additions of retzei and ha'rachaman hu yanchileinu yom she'kulo Shabbat u'menucha l'chayei ha'olamim.