When guys make heat treating ovens or forges lined with castable refractory the (dry) refractory mix is mixed with water then rammed into place and let dry. Then low heat is applied for enough time to make sure no water is left to form steam at superheated temps.
I'm thinking cinder blocks are similar. If they aren't BONE DRY then any moisture will flash into steam which can cause blowouts etc. So if you're worried do a small fire and let it go for several hours until the inside is all warm and super-dry. Or use some other means of heating the inside slowly for a long time.
That ought to make cinderblocks safe. They still might crack if you use intense heat, though.