Was the mercy seat covered with dried blood?
No. The mercy seat was not covered with dried blood. The high priest dipped his finger in the basin and touched the blood onto the golden mercy seat, rather than sprinkling gobs of blood, so it was not filthy with dried blood.
Les explains that the high priest entered behind the veil once a year and sprinkled the blood of the bullock, and later the goat, upon the mercy seat. Some people imagine it must have been filthy with dried blood, but he says they did not sprinkle gobs of blood around. Instead, the priest dipped his finger in the basin and just touched it on the golden mercy seat, presenting the blood without gobs upon the gold.
And he shall take of the blood of the bullock, and sprinkle it with his finger upon the mercy seat eastward; and before the mercy seat shall he sprinkle of the blood with his finger seven times.