After the priest killed a bull and sprinkled its blood seven times on the mercy seat, the priest killed a goat and with its blood and the blood of the bull, he sanctified the altar,
“Then he shall sprinkle some of the blood on it with his finger seven times, cleanse it, and consecrate it from the uncleanness of the children of Israel” (Leviticus 16.19).
Nothing about the tabernacle, its furnishings, or anything performed in relation to it did God order with carelessness in mind, but He wanted everything done with precision.
Therefore, the sprinkling of the blood seven times meant something to Him, and even if the priests did not know the reason for sprinkling the blood seven times, just knowing that God ordered it and that it had to be done for the consecration of the altar was enough for the priest.
Sevens in the Bible 
I’m just starting to read thru the Bible (REB) and am struggling thru Numbers having just gotten thru Leviticus. Call me silly but I noted there are numerous times sacrifices call for blood being thrown against the base of the altar and I’m thinking that after 500 or so sacrifices, or 40 years worth or, well you get the picture, it would be quite a mess and a bit unsanitary. What if anything was done about the mess?
Steve,
You have a good question, and one that I have been asked before. I have concluded one of two things:
1. The Israelites cleaned up the blood. Or,
2. The Lord took care of it.
Don