If we're just talking stalling speed then an aft cg reduces stall speed. In level flight, approaching the stall in this case, the moment produced by the wing lift about the cg must be balanced by the up or down force from the tail. If the cg is behind the wing centre of pressure then the tailplane must always be producing an up force to balance it. If the cg is ahead of the wing centre of pressure then the tailplane must produce a down force. Now, in level flight lift (from the tailplane and wing combined) equals weight. If the tailplane is producing a down force then wing lift must be greater than weight and vice versa. If the wing needs to produce more lift at a given speed then its aoa must be greater. The wing stalls at a particular aoa so, with a forward cg this occurs at a higher speed than with an aft cg.
As far as the increased tendency to stall is concerned, an aft cg tends to reduce stick force per g and the aircraft becomes less stable. So careless use of the stick may produce a g-stall. On some aircraft (like the Vulcan) fuel was moved forward to give stability and low trim drag for the cruise or back to give increased pitch control for the landing - occasionally ending up off the back end somewhere when the co-pilot forgot to stop transferring backwards. On one occasion the crew could not vacate the aircraft on the ramp until it was refuelled for fear of it tipping onto its tail!
