Customs refused to give him a reason for taking the material, saying only that it needed to be examined by forensic investigators.
Mr Blackman told the Weekend Herald last night his belongings had been returned by mid-afternoon and he was told nothing had been found.
He was also given an explanation - although he is unsure whether to believe it.
He said he was told by a Customs official that his name matched a username on an account for an internet service provider in 2007.
He said he was told someone using the account accessed objectionable material on a specific date in 2007.
Mr Blackman said the account, which was in a university student flat, was used by all the people living in the flat. He said he was not living at the address at the time the material was accessed.
Mr Blackman and journalist fiancee Imogen Crispe had attended a meeting last month at which Guardian editor Alan Rusbridger was present where the Edward Snowden NSA leaks were discussed.
He was initially suspicious this was the reason he was detained.
Mr Blackman said the Customs official told him "it had nothing to do with travels or Snowden.
"He said it's all to do with this one instance of objectionable material being flagged on the internet account."
Mr Blackman said the explanation seemed strange given he had come and gone from the country on a number of occasions since 2007 without being stopped or questioned.
He said the explanation left him wondering whether it was his public tweets from the Snowden meeting which had raised his profile.
Customs did not wish to comment last night.
