David Keogh, a cipher expert, had admitted passing on the secret memo about April 2004 talks between the two leaders in which Bush purportedly referred to bombing Arab broadcaster Al-Jazeera.
A jury continued to deliberate verdicts on a second count against Keogh and a single charge against a co-defendant, Leo O'Connor.
Keogh, 50, told London's Central Criminal Court he felt strongly about the memo, which he had to relay to diplomats overseas using secure methods, and hoped it would come to wider attention.
"The main person in my mind was John Kerry, who at the time was American candidate for the U.S. presidential election in 2004," Keogh had testified.
He admitted holding "unfavorable" views on Bush, but said he did not think publishing the document would hurt Britain's security or international relations.
The Daily Mirror newspaper reported that the memo showed Blair arguing against Bush's suggestion of bombing Al-Jazeera's headquarters in Doha, Qatar. The Daily Mirror said its sources disagreed on whether Bush's suggestion was serious.
Blair said he had no information about any proposed U.S. action against Al-Jazeera, and the White House called the claims "outlandish and inconceivable."
The document, marked "Secret-Personal," was intended to be restricted to senior officials. The memo's contents are considered so sensitive that much of the trial was being heard behind closed doors.