Only difference I can think of would be in mileage accrual. Domestic first has a higher accrual rate vs business. Seat/lounge are the same and nothing will be different. Probably just an internal fare coding issue which they unfortunately dragged you into. Air China may be selling the flight with onward service to ICN/NRT/HKG or somewhere so they code it as business instead of first.

Usually if you check the fare on ctrip or elong it'll tell you the benefits of that ticket (lounge access and the like). In the past when I've run into this situation the "business" fare will say something about how it "follows first class standards". I dont think you have anything to worry about other than the miles if that matters to you.