Should something like this reduce blackening or reduce efficiency or cause change in spectrum, or would it cause a lamp to self destruct? Is this what lifeguard lamps have already? I ask because the electrodes are usually tungsten so I'd imaging tungsten being brought back to the electrodes but I'm sure some other factors come into play as well with hid lamps

Only special highly loaded HID lamps, such as the the UHP lamps, uses halogen cycle to prevent arctube blackening.

All metal halide discharge lamps contain halogen vapours!  In many cases the thermal loading is not sufficiently high to drive a halogen regenerative cycle that keeps the wall clean, but in some types this is an integral design feature that helps to extend life and improve lumen maintenance.  Many of the short arc types for photo-optic applications have effective halogen cycles, as well as some of the more recent ceramic types for general lighting. 

Unfortunately, many metal halide lamps have tungsten transport cycles that work in the opposite direction - the halogen erodes tungsten away from the colder parts of the electrode and deposits it on the hot tip.  In these cases the halogen actually reduces life - but is a necessary evil because of their advantage in raising the metal halide vapour pressures, which is responsible for achieving better colour rendering and higher luminous efficacy than if no halogen was present.  This is one of the many reasons why the life of metal halide lamps is shorter than mercury.

The colder parts of the electrodes are not the ones where the arc root is, so how does the erosion there affect the discharge ?

The colder parts of the electrodes are not the ones where the arc root is, so how does the erosion there affect the discharge ?

The electrode lead in (support) wire is etched away and so breaks off. The same mechanism as in heavily underdriven halogen incandescent lamps...

And needed to note, in the CMH's the halide salts slowly dissolve the arctube wall material, till a leak path is formed. Another mechanism, where the halides shorten the life.

In real decent products the designers are well aware about all these mechanisms involved and so they are virtually perfectly balanced to yield maximum performance (longest life time vs maximum efficacy and color quality,...), of course the trade offs are made according the need for the given application (application requiring high color quality is tuned in a different way than another with the highest efficacy, or another where the lifetime is the dominant parameter).

The good advantage of these mechanisms is, they are (relative to others) perfectly predictable, so do not interfere that much with the reliability (so very minimum failures with reasonable maintenance group relamping schedule, with minimum wasted remaining life of the individual bulbs; do not confuse limited lifetime and reliability, these are way different things in the terms of installation management)