Can I attach a freestanding deck to concrete block garage wall?
The fact is that your deck's weight load must be carried underneath it and not using the cinder blocks supporting your garage.  That does not limit you from attaching it to the garage to provide horizontal stability and to keep movements in unison.  

You would simply need to bolt in a faux ledger to the cinder blocks.  Make sure that you have at least two posts close to the garage that you are showing as the weight bearing points for that wall.  Any good inspector should realize this.  

Note though your garage has been there for X amount of time and assuming it was made right, it probably isn't moving much.  Now build a deck and drop 4-8 posts in the ground - make sure that you are adequately putting these well below frost line and that they are big enough.  By attaching it to the cinder blocks there is a chance that your posts could cause upheaval on the garage wall. - especially with cinder blocks.