Why would you try to jack up the car at the transmission? You never want to jack the vehicle by the powertrain. I don't know what kind of jack you are using, maybe a floor jack, but a blazer has all kinds of places you could jack it up from. The front crossmember or the side frame rails is where you'd do it. Make sure you put jack stands under the frame.
Edit: I see you were probably asking where to jack the transmission when you go to unhook the trans crossmember. Jack the transmission by it's pan to get it off the crossmember. You could also support it by it's tail shafy. It's not really necessary to support the transmission when you unhook the trans crossmember but you might want to so the trans stay in line to get to the pan bolts.
I think your Blazer calls for Dexron III which this Walmart fluid probably is. You can check your owner's manual for the fluid requirements and capacity. If it were me for a few dollars more I would get a name brand fluid Dexron III or Dexron VI, and I'd also get the AC delco filter and gasket. To flush out the cooler, probably the easiest way is to unhook the return line at the transmission.
Too flush the transmission, after changing the filter out, top the fluid level, unhook the line and start and idle the engine. Shift through all the ranges for a few seconds, but make sure the parking brake is set and someone is holding the service brake firm (I'd probably put the rear axles on jack stands too for extra safety). Put it back in Park. Make sure the fluid level doesn't drop too low and when you see the line running clean fluid shut it down and hook up the line. This could flush out the whole transmission too, but you'll be going through a lot of fluid.