Here's how it would look with 8" x 8" x 16" blocks. You can use smaller size blocks, but the cost of special size blocks might not be worth it.
All sealed up like this, there would be no need to leave the arched opening, as no dirt nor animals would wander beneath it.
On the bottom I show how each end of the pre-cast beams (they come in a handy 40" length) and blocks need to be notched with the masonry saw to allow for a continuious concrete and rebar bond. All you need to make are the two blue cuts... the red section will knock out with a sharp hammer blow.
And, of course, the ends need to be formed over with plywood for the pour............. This can easily be done with a long bar clamp holding the two ends against each other on each beam, no fuss, no muss, no nails.
For this, as shown with four levels, you'll need 34 regular blocks, 8 "half" blocks, and four 40" precast lintel beams. You can form and pour your own treads and top slab.
BTW............... For an application like this, since you will be pouring all the cells solid, you can either "dry stack" the blocks, using no mortar at all... or you can use minimal mortar joints. The reason for this is to maintain as short a riser height as possible. Fat mortar joints tend to take your heights into the illegal realm. But a standard block is 7-5/8" high, and therefore makes a perfect step height if you don't blow it with big mortar joints.
The average mortar joint is 3/8". This, added to the 7-5/8" of the block makes for a full 8" inches of rise. Not a good idea when the inspector puts a tape on it. And remember, your first course of blocks has to be either a couple of inches down in the earth, or the whole course be cut off a tread thickness in height.
