How would I determine the market value or blue book value of my vehicle during a previous year (when I started using it for business)?

According to current depreciation rates, the value of a new vehicle can drop by more than 20 percent after the first 12 months of ownership. Then, for the next four years, you can expect your car to lose roughly 10 percent of its value annually.  

I recommend researching the current book value and adding 10% per year for the previous years based on the above industry standard.