How does Stitching capacitor provide shortest return current path between plane splits, as we know that it blocks DC current?
We're only talking about high frequencies here. If there is a DC connection, then there would be no need to split a plane. If you split the plane there is a loop created that has some area. That means there is inductance in the return path and a voltage can appear if the current changes suddenly (as when a signal switches). That's undesirable- it upsets the signal and emits EMI, and the reason why (rule of thumb) you should not pass a signal trace over a split in a plane if you can avoid it. 

If you cannot avoid it, then the next best thing is to make the plane appear to be one contiguous sheet of copper to high frequencies- by stitching capacitors across the trace near (say on either side) of the signal trace. 

Similar issues will occur with signals piercing multiple internal planes.