The talks involve efforts to settle investigations into the Chicago-based insurance broker's business practices.
Blumenthal wouldn't give details of the negotiations. He called the talks "difficult, but promising."
Aon is the nation's second-largest U.S. insurance broker. It has set aside $50 million to settle state investigations into insurance broker practices, but last week said it hasn't found any evidence of bid-rigging or fraud.
Officials at Aon couldn't be reached for comment, and the Illinois attorney general's office declined to comment Tuesday.