Any paint or sealer should be wire brushed off the blocks first, than additional grinding may be needed to get the wall as flat as possible. A modified thinset shuld be used to adhere the tiles to the blocks. It may be a good idea to set a ledger board on the wall to support the first row of tile, then add a few more rows using spacers to maintain a grout joint. Remove the ledger board the next day after the thinset sets up and fit cut pieces into the bottom row. Most floors are not flat or level so dont expect to set a full tile on the floor and work up from there.

tHANKS