(Read Exodus 9:8-12)

When the Egyptians were not wrought upon by the death of their cattle, God sent a plague that seized their own bodies. If lesser judgments do not work, God will send greater. Sometimes God shows men their sin in their punishment. They had oppressed Israel in the furnaces, and now the ashes of the furnace are made a terror to them. The plague itself was very grievous. The magicians themselves were struck with these boils. Their power was restrained before; but they continued to withstand Moses, and to confirm Pharaoh in his unbelief, till they were forced to give way. Pharaoh continued obstinate. He had hardened his own heart, and now God justly gave him up to his own heart’s lusts, permitting Satan to blind and harden him. If men shut their eyes against the light, it is just with God to close their eyes. This is the sorest judgment a man can be under out of hell.

For a more detailed explanation select the link below.

Exodus 9 Bible Commentary – Matthew Henry (concise) (christianity.com)

Exodus 9:10

¹⁰ They took ashes of the furnace, and stood before Pharaoh; and Moses sprinkled it up toward the sky; and it became boils and blisters breaking out on man and on animal.