The brachah, אשר קידשנו במצוותיו וציונו לקבוע מזוזה, should be recited while holding the mezuzah by the doorpost, before banging in the nails. If one forgot to recite the brachah they have lost the opportunity and cannot recite the brachah afterwards.When putting up many mezuzahs one should say the brachah before the first one and have in mind that it should also go on all the rest. So too, one should not talk until they finish putting up all the mezuzohs.