Blanche Eleanor Carnachan was born in Cambridge, Waikato, on 23 November 1871. Her parents, David Carnachan and his wife, Elizabeth Friars, had arrived in Auckland with their first five children on the Helenslee in December 1864. The Carnachans made their way to the military settlement of Cambridge, where David joined the 3rd Regiment of Waikato Militia. Seven more children were born in Cambridge, Blanche being the ninth. Her father was granted land on his discharge and ran a few cows and horses. He may have also kept a store. Elizabeth was well known for nursing around the district.
Blanche attended Cambridge School, where in 1889 she began her teaching career, first as a pupil-teacher and then as an assistant. Her father died in 1896 and the following year she left home for the first time to take charge of the one-teacher school at Wayby, south of Wellsford. In 1902 she returned to Cambridge and spent a year at Goodwood School before taking a post at Parnell School in Auckland. Her brother Robert was also teaching in Auckland, and around 1907 their mother and a sister, Jeanie, joined them to live in Epsom. From 1917 Blanche taught at Epsom School and by 1921 she was infant mistress.
During the First World War Blanche Carnachan became active in teacher politics as a member of the New Zealand Educational Institute (NZEI) and the New Zealand Women Teachers' Association (NZWTA). She was one of the most experienced women teachers in Auckland and, with a confident manner and a commanding stature, moved easily into a leadership role. In 1918 she served as president of the local branch of the NZEI and from 1924 to 1926 was national president of the NZWTA. She was involved in the campaign for equal pay for women teachers in 1924–25 after the Department of Education revised its position and reintroduced pay differentials based on gender. She also pressed for women to have access to the school inspectorate and to the top positions in larger schools.
Prior to her retirement at the end of 1927, Carnachan represented the NZEI on the 1926–28 New Zealand Syllabus Revision Committee. Besides overhauling the primary school syllabus, this committee recommended changes that would link primary education more effectively with secondary education, advocated the abolition of homework for primary schoolchildren, and advised on the secondary curriculum. The voice of its women members was heard in the recommendation that girls in secondary schools should be under 'the constant influence of women teachers'.
In 1921 Carnachan became the delegate of the NZWTA to the local National Council of Women of New Zealand (NCW). She served on the executive and was president from 1927 to 1931, when she became national president for three years. Her work on the teaching bodies and the NCW made her a prominent public figure in Auckland. In December 1926 she was gazetted on the first list of women justices of the peace and in 1928 she became an associate of the Children's Court. She was a member of the board of the YWCA from 1928 to 1931 and chaired the Seddon Memorial Technical College board of managers from 1937 to 1940. She served on the Auckland Unemployed Women's Emergency Committee in 1931–32 and on various patriotic committees during the Second World War.
After retiring from teaching, Carnachan worked voluntarily for the deaf and for intellectually handicapped children. She was a member of the foundation board of governors of the New Zealand League for the Hard of Hearing. Her interest in intellectually handicapped children had begun in the 1920s when she pressed for the government to provide better facilities and education for 'backward children'. In February 1933 she became president of the Auckland After-care Association, formed to find work and other occupations for intellectually handicapped children who had finished their schooling. The association was first affiliated to the Community Sunshine Association, which ran a school in Nelson Street, but in August 1934 reorganised itself as the Institute for the Care of Backward Children. The institute, which was the forerunner of the Intellectually Handicapped Children's Society, started its own school in Upper Queen Street in late 1934.
Blanche Carnachan had great physical presence, and a highly developed sense of community and moral responsibility. Described by her nephew as a 'strong and handsome woman…with a proud (almost imperious) upright bearing,' she played tennis and golf and swam vigorously at Little Manly Bay where the family had a holiday bach. She worked publicly for equity, morality and social justice and found public life rewarding, although somewhat at the cost of her relations with her family. In 1940 she was made an MBE. She never married, and died in Auckland on 22 March 1954.