Hello Motor, If your boiler is an old instalation, one concern is if you have any asbestos wrapped pipes in the basement, you don't want to circulate air around and stirr up any asbestos dust. Otherwise, it certainly won't hurt to circulate some air over the flue pipe. I don't know how much heat you will recover, probably not all that much.
However if you want to reclaim more heat, then you will have to increase the surface area of the flue pipe. You need to find something that when wrapped around the pipe will conduct heat [metal of some kind] and have a raised fin standing out from the flue pipe for the fan to pick up the heat. Sort of like finned tubing for hot water-baseboard heat systems
I've been thinking about what you could use, and the only item that comes to mind is the metal angle that is used to form the outside corner on Drywall [sheetrock] jobs. You could take metal shears [tin-snips] and make cuts about every inch on one side of the angle and this would allow you to wrap the solid side around the flue, secure it with a sheetmetal screw.
If you do try this, please be carefull, that piece of thin steel angle with all the cuts in it is going to be like a hand made barbed wire or Concertina wire. And when you go to wrap it around the flue pipe it will be like wrestling with a porcupine!! Get help to wrap it around the flue.
Hope this helps, Greg L