The Ospreys began the day in pole position, having won their two previous games but the Blues upset the form book with a heroic defensive display to repel the favourites.
Blues fly half Robyn Wilkins gave the Blues an early lead with a straight forward penalty before the Ospreys hit back through a try in the corner by winger Eli Norkett.
The Ospreys held the lead until the 70th minute when Wilkins slotted her second success of the afternoon in atrocious conditions. Her third penalty edged the Blues further ahead with five minutes remaining in a tense finish.
With the Ospreys forced to attack from deep, they coughed up possession and the loose ball was hacked on where Laurie Harries sealed victory by scoring in the corner with the last play of the game.
A delighted Blues skipper Gemma Hallett paid tribute to her title winning side.
"From day one I knew there was something special and unique with this squad. We've put in a lot of hard work and knew before the game against the Ospreys we could win it. We knew the Ospreys would be a formidable unit and knew we would have to front up.
"Our halfbacks Laura Prosser and Robyn Wilkins controlled the game superbly and gave the forwards a platform to work from and it was brilliant to get the win for our coach Nadine Griffiths who has led from the front."
In the earlier game, the Dragons recorded their first win of the campaign with a dogged 8-3 win over the Scarlets.
The Scarlets had pretensions of winning the title themselves but the Dragons spoiled the party with a gutsy performance in the pouring rain.
Wing Rhiannon Parker scored the only try in the corner in the 15th minute after some patient build-up play by the underdogs.
Jodie Evans reduced the arrears in the 23rd minute with a well struck penalty but her opposite number Elinor Snowsill increased the lead just a minute later for a Scarlets infringement.
In the second half the Scarlets, with Welsh internationals Sioned Harries and Catrin Edwards to the fore, piled into the Dragons but the defence held firm despite coming under a barrage of pressure.
Halfbacks Amy Day and Snowsill dictated play superbly in the latter stages, camping the Dragons deep inside Scarlets territory to guide the Gwent outfit to a deserved victory