Who is the "blonde assassin"?

One interpretation is that the "blond assassin" in the poem refers to the frost mentioned two lines previously. The frost is personified as beheading "any happy Flower," and this personification is continued in the line, "The blond assassin passes on." The frost is described as blond perhaps to compound the impression that it is cold, not just literally but emotionally. It "beheads" the flower with no hesitation or compassion. The word assassin also emphasizes this idea that the frost is a cold, unemotional killer.
A second interpretation is that the "blond assassin" refers to the sun. The word "blond" would be a more fitting description of the yellow color of the sun. The sun also might be an assassin in the sense that it looks on seemingly with indifference ("The Sun proceeds unmoved") rather than prevent the frost from beheading the flower. It thus might be said to contribute to the death of the flower, indirectly. The sun also presumably kills the frost, in accordance with the natural cycle of seasons that Dickinson alludes to in the poem, and so might be considered an "assassin" in this sense too.

In this very short poem, the "blond assassin" is winter or, at least, the frosty cold that comes ahead of the actual season. In the first stanza, the narrator says that the "frost beheads" the happy flower with "accidental power" while the unsuspecting flower plays (as in summer or early fall); in other words, the frost doesn't mean to kill the flower, but it cannot help it because this is simply what frost, what the cold, does. As a killer, the narrator refers to the frost as an "assassin" in line 5, and the adjective "blond" seems to refer to the frost's coloring. It does not refer to the frost's blond hair, but, rather, the frost is very pale and fair, like blond hair is. This paleness or fairness is what is referred to by the word "blond."