Can you hang that bag from the roof rafters instead?
Hanging something really heavy from the wall is inherently more difficult because of the twisting force that you have to overcome with your fasteners.
If you end up using toggle bolts, I would fasten your bag mount to a piece of 3/4 inch plywood with flat head bolts, and then fasten that piece of plywood to the wall with toggle bolts. Drill into the mortar joints to install the toggle bolts rather than the blocks themselves. That way, if you ever want to remove that bag mount, you can simply fill in the holes with new mortar for a nearly invisible repair.
Also, you should be aware that the horizontal mortar joints will be full of mortar whereas the vertical mortar joints will only be filled sufficiently to make them look full. That's because the vertical joints don't support any weight whereas the horizontal joints support the entire weight of the wall above them. Don't install any toggle bolts in the horizontal joints anywhere near a vertical joint. Those areas are full of mortar and the toggle won't be able to open as it should.