If regular use of diesel purge makes a noticeable difference and it turns black when you use it, its a sign you're using bad fuel.

My 240 is a perfect example. Diesel purge was never used in my 6 years of ownership, the 1 year old primary filter had only a small amount of dirt debris (no sludge or gunk) in the bottom and the rebuilt injectors have 30k miles on them. A DP this weekend made no difference at all and it turned from clear to blueish/green when the leftover diesel in the system mixed with it, never black.

If you use the proper fuel and maintain your engine then sludge and coking should never be an issue.
I only get my fuel from Sapp Brothers, a high traffic truck stop that uses quality ULSD directly from the local Suncor refinery.