The Championship side have now slid down to League One for next season, having sacked Eustace for Wayne Rooney earlier in the campaign.
Eustace had the team in the top half of the table, while Rooney lost his job in a matter of months.
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Now Blackburn Rovers boss Eustace stated: "I didn't have a clue what the scores were, I wasn't interested in anyone else. I only cared that we were doing the right things and we did. I was very confident we'd come here and be competitive and we have."
On the Blues, he added: "My job was to come in and keep Blackburn up. I didn't want Birmingham to go down, they are a club close to my heart and I have a lot of friends there. But my focus was here.
"It'd been a fantastic start to the season to leave Birmingham in sixth position. Now to keep Blackburn up has been a group effort. We've never doubted ourselves."