Wales’ Capital Region needed at least two points from the final match of the Welsh leg at BT Sport Cardiff Arms Park but they took five in a thrilling encounter.
Despite an early try from Blacker, the Blues trailed 15-5 at the break and it looked unlikely that they would make finals day in Coventry.
The Blues, led by elite pathway performance manager Richard Hodges, fought back with tries from Joe Tomlinson and Owen Jenkins but trailed 22-17 with the clock red.
However there was more drama to come in front of the 2,227 crowd and 18-year-old Blacker showed his electric pace to snatch a last gasp victory.
They will now travel to Coventry on Sunday, August 7, for the finals day and will be joined by the Ospreys who finished runners up.
Cardiff Blues made a winning start to the tournatment despite trailing at half-time to the Ospreys.
The Swansea-based region made a flying start at BT Sport Cardiff Arms Park after they were awarded a penalty try, which also saw Owen Jenkins sent to the sin-bin.
They quickly added a second try thanks to Jonathan Spratt to take control of the encounter.
But Wales’ Capital Region fought back and following an abrasive charge and audacious offload from Jenkins, Adam Thomas scooped the ball from his laces to score.
The Ospreys led 10-5 at the break but Jenkins soon powered his way over to level the scores.
Cardiff Blues remained in the ascendant and after sizing up the opposition defence, Luke Crocker dummied his way through a gap to take the lead with Ben Thomas slotting an all important conversion.
The Ospreys responded through Jay Baker but they were unable to add the extras and the Blues booted the ball out of play for a slender win.
Next up for Hodges’ men were the Scarlets who lost their opening encounter 26-12 to Newport Gwent Dragons.
The match got off to a slow start but burst into life on four minutes with the Blues grabbing three quick-fire tries.
Jenkins was first to score after sniping from the base of the ruck and powering over in the corner.
The Scarlets failed to claim the restart and were made to pay when Dane Blacker finished a sweeping move.
There was more to come and despite losing Jenkins to the sin-bin the hosts claimed a third after Ben Roach bamboozled Scarlets’ defence to release James Beale.
The try gave Cardiff Blues a 15-0 half-time lead but the Scarlets responded with scores of their own from Nathan Edwards and Aron Evans.
They threatened to draw level but the Blues snuffed out any hope when a thumping tackle and turnover from Luke Crocker opened the door for Ben Roach to seal a 22-12 bonus-point win.
Cardiff Blues made the perfect start to their final encounter with the Dragons coughing up the restart and Dane Blacker scoring on 11 seconds.
However the Rodney Parade outfit took control of the clash with tries from Jonny Lewis, either side of a Lloyd Lewis effort.
The Dragons were dealt a blow when Ethan Lewis was sent to the sin-bin and the Blues took full advantage as Jenkins burst through for his third try of the evening.
And to make matters worse Jonny Lewis caught the Wales 7s international with a swinging arm in the process of scoring and was duly sent to the sin-bin.
With the Dragons down to five-men Joe Tomlinson gave the Blues the lead but it lasted a matter of moments as Chris Levesley scored a stunning individual try to regain the lead.
However Cardiff Blues refused to throw in the towel and with the clock at zero they created space for Blacker who needed no second invite to show a clean pair of heels.
Cardiff Blues 17 Ospreys 15
Scarlets 12 Newport Gwent Dragons 26
Newport Gwent Dragons 19 Ospreys 21
Cardiff Blues 22 Scarlets 12
Ospreys 24 Scarlets 5
Cardiff Blues 24 Newport Gwent Dragons 22
Cardiff Blues squad: 1 – Adam Thomas, 2 – Ben Roach, 3 – James Botham, 4 – Luke Crocker, 5 – Joe Tomlinson, 6 – Cameron Lewis, 7 – Owen Jenkins, 8 – James Beale, 9 – Dane Blacker, 10 – Geraint Walsh, 11 – Ben Thomas, 12 – Declan Williams