An earlier district court decision found Slater did not qualify as a journalist and would have to reveal who gave him the documents and computer hard drive on which his posts were based.
Mr Miles said a series of posts to the blog during 2011 and 2012 showed Slater had developed a pattern of work which involved receiving information and then publishing it. It meant he should qualify for the protection enjoyed by journalists, he said.
He said if the High Court found Slater was a journalist, it would then have to weigh public interest in revealing sources against the negative impact of doing so. In the case against Slater, he said Mr Blomfield had no wider interest to the public and allegations raised in the blog posts had been considered and dismissed by the authorities. While the disclosure of sources might put some off passing on information, it could also dissuade those passing on information which held little public interest, he said.
Asked for precedents, he said: "I know of no case which would go so far as to say this internecine shareholder fight in a relatively unimportant private company is entitled to the protection of anonymity."
Mr Miles also responded to questions from Justice Raynor Asher, who asked whether Slater would have to reveal his sources if he wanted to rely on a defence of "honest opinion". Mr Miles said: "I'd be surprised if Mr Slater could run a successful defence without disclosing."
Slater said he was entitled to the same protection afforded to traditional media outlets in gathering information.
