Hello Steve. The device block size is 512 bytes and is not changeable. It is the smallest IO that a host can address(read/write). Your oracle db_block_size of 8kb is already in alignment with the 512 byte block size. Also, for buffered file IO on jfs2, it does not matter if you use a 512byte or a 4KB block size.  Regards. 
One of our clients has devices allocated from EMC VMAX to their Oracle RAC and have queried the block size paramater.  I have attached a screenshot to show an example and my question is in relation to the Device Block Size value of 512 - is this bytes or kilobytes. The application team using the device have set IO Size at 8KB and are curious as to why the Device Block Size at the array level is 512. Is this something we can change at the array level to align it with the application IO Size of 8KB. Or is this created when configuring the disk groups.
VMAX Block Size