Q: Can a person ask for a blessing from a deceased tzaddik, "righteous person"? Can a person ask for a blessing from anyone, besides G-d?
A:It is certainly permissible to pray to G-d for a blessing at the gravesite of a tzaddik, in the merit of the tzaddik. In addition one may directly address the soul of the tzaddik, beseeching him to arouse mercy in heaven.
The Zohar writes that the tzaddik can more effectively arouse Divine mercy after his death than previously. (See chapter 27 in Igeret Hakodesh of Sefer Hatanya).
When one goes to pray at the gravesite of a tzaddik, it is as if he is entering into yechidus ("private consultation") with that tzaddik. Thus, it is customary to recite chapter 33 in Psalms at the gravesite. This chapter begins with the verse, Ranenu tzaddikim baHashem, "May the tzaddikim rejoice in Hashem." The verse refers to tzadikkim in the plural form, which is understood to refer to the union of the lower tzaddik — the soul of the person who comes to pray at the gravesite — and the higher tzaddik — the deceased tzaddik.
When a person prays at the gravesite of the tzaddik, the two souls unite to arouse Divine mercy and to bring down blessing.