Blacklisting local device /dev/sda for RHEL 6 (Blacklist all local devices) 

When adding SAN to RHEL 6 hosts,  The local device /dev/sda (local boot disk) must be blacklisted in /etc/multipath.conf.

DO NOT UPGRADE KERNEL WITHOUT BLACKLISTING THE LOCAL DEVICES (/dev/sda or other boot devices) 

If the local device (in most cases sda)  is NOT blacklisted and new Kernel patches are applied,  Dracut will include multipath configurations  to
to initramfs (initial ram disk file)  and will update boot configurations in /etc/grub.conf  and host will not properly boot as the local boot device (/dev/sda)  is not multipathed. 
To back out the changes to the kernel, see the red hat article to revert.
https://access.redhat.com/site/solutions/320053
If you start multipathd and the local device sda is NOT blacklisted you can still fix changes as long as you do not  update the kernel or reboot the host.
Edit multipath configurations
UPDATE:     After further research it is better to blacklist the local controller of the host.   just in case SDA is not the default boot device.  The below example blacklists for
BOTH HP and DELL servers. ( Both device configurations can remain in multipath.conf) 
Add device sda to the blacklist section of /etc/multipath.conf
blacklist {
             # wwid 26353900f02796769
             devnode "^(ram|raw|loop|fd|md|dm-|sr|scd|st)[0-9]*"
             devnode "^hd[a-z]" 
             device {
                          vendor "DELL"
                          product "*"
             }
             device {
                         vendor "HP"
                         product "*"
}
Edit /etc/multipath/bindings file and remove mpatha <WWN> and save the file.
Restart multipath and you can safely reboot the host.