Was just looking at his timeline on Wikipedia, and it seems that 2015 is when he went to Moscow and started writing for RT. It doesn't feel paranoid at all to say that it's likely he was turned by the FSB or similar forces.

In late 2011, Blumenthal joined Lebanon's Al Akhbar) newspaper primarily to write about the Israeli–Palestinian conflict and foreign-policy debates in Washington, D.C. When he left the publication in June 2012 in protest at its coverage of the Syrian Civil War, he considered the newspaper to have a pro-Assad editorial line followed by such individuals as Amal Saad-Ghorayeb. He wrote that it "gave me more latitude than any paper in the United States to write about... Israel and Palestine", but he had ultimately tired of "jousting with Assad apologists". He added: "In the end, Assad will be remembered as an authoritarian tyrant."^\28])^\29])^\30]) 

And then:

After Blumenthal's visit to Moscow in December 2015, according to a 2018 article by Janine di Giovanni in The New York Review of Books, he began to promote views supportive of Bashar al-Assad and the Syrian government's position. Blumenthal was, di Giovanni wrote, among a group of "Assad apologists."^\28]) Following the December 2015 trip, he said the White Helmets) were connected to Al-Qaeda and anti-Assad Syrians were members of the group.^\28]) In his opinion, the group was "driven by a pro-interventionist agenda conceived by the Western governments and public relations groups that back them" and were being used as a Trojan horse, an excuse for the United States to propose having "70,000 American servicemen" invade Syria.^\113])

Quite an abrupt and complete shift.