Not all Brahmins have mongolian blood. The 'purohit' however you spell it, are specifically priests who are forbidden (completely) to intermingle. Could there have been a few mixing (yes), but they were demoted to other castes immediately, or they kept their last name, but left the 'tribe' so to speak. Not all (actually most of them aren't) Nepalese brahmins are purohits/pandits. In the Brahmin community, you'll notice some (mostly from the gorkha region) have very, very light skin (as compared to other Nepalis). Even then, could there have been mongolian mixing? Yes. But nowhere close enough to make a phenotypic change.