David Keogh, 50, a cipher expert, had admitted passing on the secret memo about April 2004 talks between the two leaders in which Mr. Bush purportedly referred to bombing the Arab television station Al-Jazeera.
A jury continued to deliberate on a second count against Keogh and a single charge against co-defendant Leo O'Connor.
Keogh 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 President Bush, but said he did not think publishing the document would hurt Britain's security or international relations.
The Daily Mirror newspaper previously reported that the memo showed Blair arguing against Mr. Bush's suggestion of bombing Al-Jazeera's headquarters in Doha, Qatar. The Daily Mirror said its sources disagreed on whether the president'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, which are considered so sensitive that much of the trial is being heard behind closed doors, have not been directly referred to by counsel or witnesses in open court.