Manchester City, who slipped to second place in the standings following Chelsea's 6-0 thrashing of Aston Villa on Saturday, came into the game on a fine run of form but they fell behind when Steph Catley put the Gunners ahead in the 14th minute.
City's teenage keeper Khiara Keating then stepped into the spotlight, conceding a penalty for a foul on Cloe Lacasse but saving Kim Little's spot kick in the 20th minute, and it was not the last time she was the centre of attention.
Chloe Kelly thought she had made the most of that miss when she drove home the equaliser for City in the 72nd minute, but Keating completely misjudged a late long ball that bounced over her, allowing Blackstenius in to nab the winner.
After four straight defeats, Bristol City finally tasted victory in the WSL as they came from a goal down to beat West Ham 3-2 away.
Everton's Aurora Galli scored from the spot in the 85th minute to give her side a 1-1 draw away to Tottenham Hotspur, while Liverpool's excellent start to the season continued with a 2-1 win over Leicester City.
Chelsea top the table on 13 points after five games, three ahead of Manchester City, Spurs, Liverpool and Arsenal, while Manchester United can go second in the table if they beat Brighton & Hove Albion in Sunday's late game.
