“Wash me with Hyssop” is a direct reference to the blood of the Paschal lamb-
Nah....couldnt be. A) The Paschal lamb had no atonement. There was no covering of sin or forgiveness of sin with the Paschal lamb, David knew that. The Paschal lamb was a sacrifice of the Egyptian idol and god, the lamb. And B) David would be referring to the cleansing water sprinkled from the hyssop branch. In Numbers 19
18 Then a man who is ceremonially clean is to take some hyssop, dip it in the water and sprinkle the tent and all the furnishings and the people who were there.....The person being cleansed must wash his clothes and bathe with water, and that evening he will be clean.
And David says: 7 Cleanse me with hyssop, and I will be clean; wash me, and I will be whiter than snow.
David is clearly refering to the sprinkling and purification process in Numbers 19.