The magnitude of Arsenal's achievements will be felt in days to come but this was a sensational night in Portugal where they showed what they had always promised they were capable of.
They began the season amid fan unrest, with former manager Jonas Eidevall's subsequent resignation, but Slegers – who initially took over on an interim basis before being named permanent head coach in January – has taken on the baton in impressive fashion.
She steadied the ship and guided Arsenal through a tricky group stage of the Women's Champions League, before exceeding expectations to lead them to the final.
They have gone on to become the first club to win the Women's Champions League having started the campaign in the first round of qualifying.
She wanted her side to show courage, despite many writing them off in the days before, but surely could not have imagined just how glorious the night would be.
Atmosphere had been building in Lisbon throughout the day with Barcelona fans, following their usual routine.
Thousands of supporters, in blue and red striped shirts, gathered for a march outside Estadio Jose Alvalade hours before kick-off, and they made plenty of noise as their superstars arrived by coach.
They had been here before and were expecting a similar result.
But on the other side of the city, Arsenal supporters were gathering on Pink Street, sporting shirts with Leah Williamson, Alessia Russo and Little's names on the back, as they went through their songbook.
Arsenal co-owner Josh Kroenke flew in from Denver and was alongside executive vice-chair Tim Lewis, managing director Richard Garlick and director of women's football Clare Wheatley.
A picture of the noticeboard displaying messages of support sent in by fans at the club's Sobha Realty Training Centre was brought to Lisbon.
And even members of the 2007 Arsenal winning squad were in attendance, many having joined Slegers' team for a lunch this week to reminisce on their special occasion.
They will have been encouraged by Arsenal's first half but frustrated they could not finish any of their chances. In the second half they rode their luck a little but Blackstenius provided the clinical finish when it mattered.
When the full-time whistle went Arsenal's players and staff raced onto the pitch to celebrate. Many collapsed to their knees, others jumped into the arms of others. It was an explosion of pure joy and emotion.
European champions again. Defying the odds again. It has been the Arsenal way.