How can I securely hang a very heavy object on wall made of hollow bricks?
There is a class of anchors called "hollow wall anchors".  Most of the time these are used for sheetrock or plaster walls so people don't think about them when it comes to brick, but many of them will work in hollow bricks.  There are many types and should be available at any home store.



Sample toggle bolt

The challenge will be making sure you hit a "hole" in the brick.  There are different anchors for hollow bricks and for solid bricks, so it probably make sense to have both on hand.  If you drill a hole and hit a solid portion of brick, use the solid anchor.  If you hit a hole, use the hollow anchor.

When hanging a large item like a blackboard, you may be able to adjust your hole spacing to make sure you always hit a hollow area.