Slaying a Titan with a Crucible seems to result in a blue light to course throughout the demon's body. The Codex doesn't mention this light and it seems to only manifest once the Slayer removes the Crucible hilt while leaving the blade embedded within the Titan (the same thing happens to the Icon of Sin later). Samuel Hayden does mention "If the Crucible blade is removed, the Titan will rise again", so perhaps keeping the blade embedded while taking out the hilt ensures the demon remains dead. In the cutscene, the Titan seems to move briefly when the blue light manifests before freezing still again.
After killing the Demon-Spitter (another Icon of Sin) in Final Doom's TNT Evilution, the marine hero notices a "blue light" glowing within the skull of the demon:
"Suddenly, all is silent, from one horizon to the other. The agonizing echo of Hell fades away, the nightmare sky turns to blue, the heaps of monster corpses start to evaporate along with the evil stench that filled the air. Jeeze, maybe you've done it, have you really won?
Something rumbles in the distance. A blue light begins to glow inside the ruined skull of the demon-spitter."
It's possible that the blue light coursing through the Titans of Eternal on their death is referencing the blue light glowing inside the Demon-Spitter in Final Doom on its death. An Icon is considered a Titan, and so would the Demon-Spitter by extension. Of course, the marine hero in Final Doom didn't kill the Demon-Spitter with a Crucible blade but it's still an interesting parallel that could be intentional.