Which Decepticon seeker transforms into a blue jet?

The Decepticon Seeker that transforms into a blue jet is Thundercracker.

Thundercracker is one of the original Decepticon Seekers introduced in Generation 1. His alternate mode is an F-15 Eagle-style fighter jet, and his distinctive blue-and-gray color scheme separates him from the other members of the first Seeker trio. He commonly operates alongside Starscream and Skywarp, making the three easy to compare in the original cartoon and toy line.

The Seekers share a similar robot design because they were built from the same basic animation and toy model. Their colors and personalities provide the main visual distinctions: Starscream is predominantly red and white, Skywarp is black and purple, and Thundercracker is blue. Thundercracker is often depicted as a powerful aerial warrior who can generate destructive sonic booms.

Later Transformers continuities redesign characters and introduce additional Seekers, so colors and vehicle details can vary. For the classic identification implied by this question, however, the blue jet is Thundercracker. Acid Storm is usually green, while Starscream and Skywarp have their own familiar color patterns.