The word αἵματος (blood) is in the genitive case due to its subordination to the preceding noun, ῥαντισμὸν (sprinkling), a relationship reasonably well represented by the English "of" (i.e "sprinkling of blood"). I'm not sure exactly why you think it should be in the accusative case, but I can imagine at least two approaches that might lead to such conclusion:
This is part of a prepositional phrase headed by εἰς, which takes an accusative object.
"Blood" is what is "sprinkled", i.e. the direct object.
Both are apposite considerations, so I will respond in turn:
Case is determined (with some exceptions) by the syntax at the lowest level of subordination. Here a noun (αἵματος, blood) is subordinated to another noun (ῥαντισμὸν, sprinkling). Generally, such a relationship is expressed by putting the subordinated noun in the genitive case. On the other hand, the word ῥαντισμὸν (sprinkling) is indeed in the accusative case, as expected after the preposition.
This is precisely why some would label this an objective genitive. The noun "sprinkling" contains a verbal idea. "Blood" is the object. However, because ῥαντισμός is in fact a noun (derived from the verb ῥαίνω = to sprinkle), it takes a genitive "object." Such is the nature of the genitive.