Abrahams will become BLM's strategic planning director with a brief to strengthen strategic thinking across divisions including BLM Media, Quantum New Media and creative agency Flint.
He replaces Graham Hawkey-Smith who is leaving BLM to launch his own agency. However, Abrahams' role will have more of a group focus.
Abrahams has been at Carat for four years developing communication strategies for clients including Cahoot!, News International, Heinz and Madame Tussauds.
Prior to joining Carat, Abrahams worked at Bates Dorland for ten years, rising to media board director.
Paul van Barthold, the managing director of BLM, said: "We see our strengths as a company matching Guy's. We are a choice for clients that don't feel the offering of big agencies is right for them and have a heritage in innovative solutions and planning. Guy has a very good track record on that."
Abrahams, who joins BLM this week, said: "BLM has a creative approach to media and it's an opportunity for me to join a company where clients tend to be challenger brands. If you want to provide bold solutions it's easier to do if you are David rather than Goliath. It will be good to use insight and lateral thinking on outwitting the competition."
BLM Group employs 80 people and has an income of £7.2 million. Key clients include Domino's Pizza and Thomas Cook.
Earlier this month, it won the £5 million media planning and buying account for film distributor Pathe.