First of all, we don't say "ma b3rif" ; we say "ma3rafsh" or "mish 3aarif(male)/3arfa(female)" and for "really", we would add the word "el7a2ii2a" before or after any of them.
We also use fe3lan:
مش عارفة فعلاً
أنا فعلاً مش عارفة
and beSaraa7a:
بصراحة مش عارفة
مش عارفة بصراحة
Although beSaraa7a means "honestly" more than "really", but I think the meaning and usage are close to "really don't know".