The series started last Monday with 5m viewers but was beaten by BBC One's 'Waking The Dead', which registered 6.1m.
Last night 'Waking The Dead' extended its lead, reaching 6.7m viewers and picking up a 29% share of viewing.
Despite the audience decline 'Blue Murder', which is sponsored by Toyota, gave ITV1 a 19.5% share of viewing and put it way ahead of the three other channels in the 9pm-10pm slot.
Channel 4 was closest behind with 1.4m viewers for the first in a series of five 'Daredevil' documentaries examining what drives some people to risk their lives in dangerous feats.
Last night's 'The Human Bird', about a man who leaps off mountains in a winged costume, achieved a 6.1% share. It was watched by 112,000 on Channel 4+1.
Five's Royal Navy documentary 'Warship' brought in 1.1m viewers and a 4.6% share.
BBC Two's new six-part series 'Design For Life' got off to a disappointing start. Copying the format of 'The Apprentice' to the world of design, with Philippe Starck in the Sir Alan Sugar role, 'Design For Life' was seen by 930,000 viewers and managed a 3.9% share of viewing.
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