After the break, both sides spurned good chances, with Katie McCabe darting in from the right, and shooting from a tight angle straight at Szemik. Back came West Ham, who fashioned a chance of their own as Asseyi twisted and turned inside the box before her effort crept just wide of Daphne van Domselaar’s near post.
A quadruple change with just over 30 minutes left signaled Renee Slegers' intent to turn the tide, as Beth Mead, Blackstenius, Kyra Cooney-Cross and Kafaji were introduced.
The substitutions almost paid an instant dividend too, when Russo’s clever reverse pass played in Blackstenius, who was only denied by a quality save from Szemik.
However, midway through the half, the game’s turning point came when Caldentey sent Szemik the wrong way from the spot after Blackstenius was brought down by Amber Tysiak to the left of the area.
Substitute Emma Harries then climbed off the bench as the Hammers sought a route back into the game. And the England U23 international almost had an immediate impact, rising above a Gunners defender and glancing Riko Ueki’s cross straight at van Domselaar.
West Ham suddenly had more of a presence in the final third, but in committing numbers forward the visitors were also presented with opportunities.
Moments after entering the fray in place of Shannon Cooke, Dagný Brynjarsdóttir came agonisingly close to snatching a point as her back-post header was brilliantly kept out by van Domselaar.
With the Irons chasing a leveller, gaps on the break appeared and the visitors made it two soon after as Blackstenius embarked on a marauding run from halfway and picked out Kafaji for a first-time finish to secure maximum points.
West Ham United: Szemik, Tysiak, Cooke (Brynjarsdóttir 81), Zadorsky, Denton, Gorry ©, Siren, Mengwen, Piubel (Paví 81), Ueki, Asseyi (Harries 72)
Subs not used: M.Walsh (GK), Smith, Bergman Lundin, Sáez, Houssein, S.Walsh
Booked: Asseyi
Arsenal: van Domselaar, Fox (Reid 90), Codina, Catley, Caldentey, Little © (Cooney-Cross 59), McCabe, Maanum (Kafaji 58), Wälti, Foord (Mead 58), Russo (Blackstenius 58)
Subs not used: Zinsberger (GK), Williamson, Kuhl, Hurtig
Goals: Caldentey 71 (pen), Kafaji 89
Booked: Caldentey
Referee: Tom Parsons