Blurr was a lot smarter than others gave him credit for. His speed-matrix not only made him go faster physically it also increased his processor speed. It made him more perceptive, he noticed more things than others missed. Like Longarm.
Blurr had known from the start that Longarm wasn't all he pretended to be. It had taken a bit longer for him to figure out that he was Megatron's spy, Shockwave. The moment is still clear in his memory, he remembered feeling betrayed and thinking that he had to tell Ultra Magnus before Shockwave got them all killed, and that Wasp needed to be released and have his name cleared. He'd gotten all the way to Ultra Magnus' office door before thinking that there had to be a reason a mech as smart as Shockwave had sided with the Decepticon’s.
He'd retreated to his quarters to think the different sides of the war out. He hadn't liked the answers he'd come up with. He concluded that the Autobot's where run by a bunch of bureaucratic bullies bent on subverting the population that wasn't them. He knew he hadn't really had much choice in being an Autobot because of his creators and the amount of credits spent on his speed-matrix. He wondered how many mechs were or had been in the same situation. Forced into service, paying back the debt incurred by their creators. He decided in that moment of clarity to help Shockwave when and where he could.
When Shockwave revealed his alter ego to the Autobot’s he'd been ready to murder the mech for his stupidity. After escaping Thundercracker and Skywarp, and managing to run across the galaxy on space debris to get back to Cybertron. He'd reported his 'findings' to 'Longarm Prime' and said that they would be able to uncover the identity of the traitor with a few scans. He'd added "You need to stop being stupid with who you reveal your true identity too, I can't protect you forever". The look on his face was priceless. Longarm had shifted into Shockwave, grabbed him by his neck, pushed him into a wall and demanded an explanation. The answer was obviously unexpected and it had taken a lot to get 'not dead'. They'd come up with a plan to convince Optimus' group that Longarm was a double agent for them and not Megatron, they had fallen for it far too easily.
After that, the war had gone rapidly downhill for the Autobot’s. They lost ground and mechs faster than they could replace them, and more mechs joined the Decepticon’s when it became obvious they could actually win. The Decepticon’s had taken the Cybertron Space bridge nexus and decimated the Elite Guard. Blurr had been 'captured' in that attack and taken to what he assumed was a holding cell. He was given fuel and mostly left alone, presumably on orders because he knew other mechs captured were being tortured for information. He didn't know what had happened after that, no one here told him anything, until the Decepticon guards started celebrating their total victory over the Autobot’s. The Decepticon’s had won the war.
Not long after that Shockwave had come down to retrieve him. They'd spent the next ten Joors in Shockwaves quarters alternately interfacing, fuelling and talking (in that order). Blurr was sure it would have been longer except that Megatron demanded an explanation. Blurr would forever remember the expression on his new leaders face when Shockwave finished talking, he nearly hadn't been able to restrain his laughter.
Megatron decided that Blurr would stay with Shockwave and act as his personal assistant until Megatron was sure he was really on their side and wouldn't turn around and help the remaining Autobot’s, who were kept online to do all the manual and menial jobs Decepticons thought were now beneath them. Other Decepticon’s hadn't responded well to the orders that Megatron would not support any sort of slavery besides manual labour, that if he caught any of his soldiers forcing a mech into interfacing, they themselves would become pleasure slaves.
He hadn't seen many Autobot’s working as a PA to Shockwave nor did he care to, the looks he got from them said enough.
He was happy with how things had turned out so far.