I would treat this like a hole in a drywall section. I would try to find a backer board or something to act like a back wall to hold the material that I will trowel into the hole.
On drywall, I use a 1 x inserted in the hole and screw it onto the back of the wall and there fore I have a backer to apply mudd and tape.
On your cider block, you can insert a blocker and then fill the hole with latex concrete patch and then texture it a bit to match the haydoc block. If you cannot find some sort of backer, then just fill the hollow block with cheap concrete then latex concrete patch on the exterior.
I like latex concrete patch because it does adhere well and does have some flexibility to it. I don't know how but it works great for patching bad concrete walkways and the likes.