She first entered politics when she was elected to Aberdeen City Council as an SNP councillor in the Hilton/Stockethill ward, in the Aberdeen North constituency in the 2007 Aberdeen City Council election topping the poll in her ward with 1,761 first preferences. Her brother, John West, was also elected for the Hazlehead/Ashley/Queens Cross ward in the same election. She was re-elected in 2012 Aberdeen City Council election with 823 first preferences taking the second seat in the ward She then became the Convener of the SNP group in Aberdeen City Council.
At the 2015 general election, she became the Member of Parliament for Aberdeen North. She succeeded Frank Doran, of the Labour Party, who had announced in October 2013 that he would stand down at the next general election. She won the seat with 24,793 votes, 13,396 more than the Labour Party candidate Richard Baker, and became SNP Spokesperson on the House of Lords. In April 2016, she drew attention to the plight of unaccompanied refugee children during a Commons debate. She was elected to the Scottish Affairs Committee in 2015. In July 2016, she spoke out on the importance of making Parliament more family-friendly after being censured by clerks for holding her sleeping two-year-old daughter in a committee hearing.
She retained her seat at the 2017 general election. Following the election, she became Deputy Leader of the SNP Westminster Group and SNP Westminster Spokesperson on the Economy. She was the first woman to lead for her party in the House of Commons on the Economy.
Blackman held the seat at the 2019 general election with a majority of 33.9%, the largest percentage majority in Scotland. In January 2019, she became the SNP Spokesperson for the Constitution at Westminster, a strategic role leading on the constitution in preparation for an independence referendum.
In July 2020 Blackman announced that she would be stepping down as the deputy leader of the SNP Westminster Group, saying “Like many others, I’ve struggled with the impact that lockdown has had on my mental health. In order to prioritise my constituency and my family, I have made the difficult personal decision to step down from my Leadership role. I strongly believe that people must be able to talk openly about mental health issues, which affect so many of us”. On 7 July it was announced that Kirsten Oswald was to be her successor. On 24 September 2020 she spoke more about suffering with depression.
In January 2021, Blackman publicly criticised former party leader Alex Salmond, labelling him an “awful human”.
In November 2021, Blackman offered unreserved apologises after using the suicide of Rose Paterson, wife of the Conservative MP Owen Paterson, to make a political point about the lobbying scandal in which Paterson was involved in at the time. Blackman tweeted “Missed your bus because your wife committed suicide and so you were two minutes late to your job centre appointment? – SANCTIONED. Lobby the Government on behalf of companies who pay you £9k a month? oh, you poor lamb, no sanction for you.” Rose Paterson committed suicide by hanging in June 2020. The tweet has since been deleted. She later tweeted “After some reflection, I have deleted a tweet I made earlier. I offer my unreserved apologies for tweeting it, particularly to anyone who may have been upset or offended. I’m sorry.”.