Blakeman, who plans to take up an agency role in Singapore, is due to leave PHD around mid October.
Spencer was formerly based in the PHD office in London, before moving to Manchester this summer. He has a total of 10 years' experience in the industry, including posts at Carat and the Manchester Evening News.
PHD managing director Morag Blazey said: "PHD Manchester is in great shape thanks to Steve and is being passed to a genuine innovator in an already dynamic market."
We are thrilled that Jason has settled in so well in Manchester and is already making an impact in the business."