A Blue Lantern Superman Would Be Clark Kent's Ultimate Form
While Superman is one of the most powerful beings in the entire DC Universe, one piece of alien technology would make him unstoppable: a Blue Lantern power ring. Where the Green Lantern Corps are powered by Will, the Blue Lanterns channel the power of Hope. As a beacon of justice and equality whose iconic emblem is derived from the Kryptonian symbol for "Hope," few candidates would be more worthy to join the Blue Lanterns than Superman.
A number of comics have shown the Man of Steel wielding different power rings - including Black, White, Yellow, and Green rings- though these are either temporary or alternate-universe tales, such as Superman: Last Son of Earth. However, there is an in-universe explanation for why he didn't become the Green Lantern of Sector 2814: under Guardian law, Green Lanterns must descend from the sector they protect. As a Kryptonian, this precludes him from guarding Earth. However, this rule need not apply to the Blue Lanterns, who would presumably be looking for new recruits after Relic destroyed all but Saint Walker - but what would a Blue power ring even offer the Man of Steel?
Created by Ganthet and Sayd following their exile from the Council of Guardians, Blue Lanterns share many of the same abilities as Green Lanterns - flight, force-field generation, light constructs, etc. - apart for a few differences. Most notably, they require close proximity to a Green Lantern ring to reach their full potential, but when fully powered are able to boost or nullify other Lanterns, heal almost any physical injury, and even reverse the aging of dying stars. While Superman already has many of the abilities granted by power rings, he would likely benefit most from the power to heal others and restore dying stars. If, furthermore, Blue Lanterns are dependent on the user's ability to inspire Hope in others, then Superman's established status as an inspiration to heroes and citizens across the universe would surely offer near-limitless power, as well as super-charging his ring-bearing allies.
It is unclear whether a power ring's force field would protect Superman from the effects of Kryptonite, but one character might provide an answer: Sodam Yat. As a Green Lantern from the planet Daxam - an off-shoot Krypton race - Sodam Yat receives similar powers to Kryptonians in the presence of a yellow sun, though he is vulnerable to lead rather than Kryptonite. In Green Lantern Corps #18 (from Peter J. Tomasi, Patrick Gleason, and Jamal Igle), Sodam Yat is near-fatally poisoned in a fight with Superboy-Prime, though he is kept alive by his ring's protective aura - meaning that he will die if he takes it off. The implication is that power rings can negate the effects of deadly substances and materials, though perhaps a Blue Lantern might require a Green Lantern supercharge to do so.
Although the Blue Lantern Corps are rarely used by DC, members of the Superman family have wielded the Blue light of Hope outside of mainstream continuity: Future State: Superman: The House of El (from Phillip Kennedy Johnson and Scott Godlewski) featured Rowan Kent, a distant ancestor of Superman, who is the Blue Lantern of Earth in the 30th century; and Nightwing: The New Order (Kyle Higgins and Trevor McCarthy) gave Lois Lane a Blue power ring. Hence, while it is unlikely that DC will ever permanently make Kal-El a Blue Lantern, perhaps there is a hint of recognition that no one would be more worthy to channel the cosmic power of Hope than Superman.