I’m not sure how to make it spawn only on those blocks. I don’t want it spawning everywhere; only on those blocks, so that the main character can get the object once it spawns then it is destroyed.
You could create another block under the place where you want to spawn it. Use always to extract the x axis of the block and send it to the x of the spawner.
If the boss drops the item, just have it send a message to a ground block telling it to emit.