Ihaia West's scampering run for the try beside the posts put the Blues on top at halftime but it was Ranger's explosive run to draw two defenders, and his assist while lying on his back, which created it.
Earlier he was responsible for two turnovers as he stuck his head in areas most midfield backs wouldn't consider.
It took a while for Ranger's teammates to get in the same groove. Wing Tevita Li had a night to forget - penalised in quick succession in the first half, the second whistle bringing a yellow card for what was considered a professional foul on defence; he resembled a revolving door in midfield when spinning and speeding up, rather than hindering, the run of first-five Santiago Gonzalez Iglesias.
Blues fullback Lolagi Visinia also had the misfortune of kicking the ball straight into Steven Luatua when on defence, although his loose forward teammate would consider himself more unlucky, hit as he was on his exposed back at point blank range.
As the Jaguares, roared on by a vocal minority of their blue and white fans in a crowd of 10,300, drew within touching distance at 21-16 thanks to the converted try from Gonzalo Bertranou, Ranger's grubber kick picked up by Visinia and transferred to replacement second-five George Moala went close to knocking the stuffing from the visitors.
There was no try, but territory was maintained, and West kicked the resulting penalty. It was about as close as the Blues got to being clinical.
Given the Jaguares' penchant for throwing the ball around - often at their own cost - this wasn't the free-flowing spectacle many had predicted.
For them it was a disappointing result after they exerted so much early pressure. For the Blues, it was a precious victory - their second after five games after their opening success over the Highlanders.
It was also their first win at North Harbour Stadium since 2013, a fitting way for lock Josh Bekhuis to celebrate his 100-game milestone, a relief as far as the Blues are concerned, after last year's celebrations for Jerome Kaino's 100th and Keven Mealamu's record-breaking 163rd Super Rugby appearance were ruined by the Lions' 13-10 victory here.