Boot Camp script broke after El Capitan update

You can't set the bootable disk with bless if System Integrity Protection is enabled. This is because bless writes to NVRAM, which is prohibited by SIP. This is why System Preferences → Startup Disk is able to set the startup disk, but other binaries are not.

To disable System Integrity Protection, you can do the following:

1. Boot into the Recovery HD by restarting whilst holding ⌘R.
2. Open Terminal (from the Utilities menu).

3. Run the following command in Terminal:

   csrutil disable
   

4. Restart.

^{Source: Disable OS X El Capitan Rootless and permit write actions to System Integrity Protection locations}