At last Pharaoh had had enough. It took the death of his firstborn son, but he couldn't get rid of the Israelites soon enough. In so many words, he said "Skedaddle! You, your wives, your children, your livestock, your belongings. Don't let the door hit you on the way out."
At the end of his frantic plea for Moses and the Israelites to leave, Pharaoh tacks on a strange request "and bless me."
After God had devastated Egypt's livestock, crops, people, water--why did Pharaoh think Moses would agree to bless him?
Why did Pharaoh want a blessing from this stranger's God?
I think it might be as simple as this: In asking for a blessing, Pharaoh laid his last bit of pride on the line. The word for blessing is connected to the word for "kneel." The author of Hebrews says that the greater blesses the lesser.
By asking for a blessing, Pharaoh admitted that Moses and his God were powerful that Egypt and its gods.
Here's my great, "spiritual" takeaway: Sometimes a blessing is the best revenge.