You can probably get a decent rental from Yorktown Cycles (914-245-5504), though it won't be convenient to go from the city to the bike shop and then to Blue mt. Best be is to rent a car by the hour (zipcar.com), assuming you're coming up from the NYC?
Down Cycles (914-827-9570) and Briarcliff Bikeworks (914-762-7614) are more in a straight line approach from the city, but I don't know if they offer rentals.
No guide is necessary, as long as you have time to find your way out. And post up when you're coming, and you can more than likely find someone to guide you. Lots of nice folks in these parts