It was a case for too little too late, as Blacks replacement scrum half Martin Roberts scored a late try to narrow the gap to a two point deficit in the closing stages of the game.
Neath Assistant Coach, Gruff Rees commented that with the title already decided midweek, it may have taken pressure off the Blacks. He said: "No side likes conceding a league double, and that is especially the case when it is Cardiff. However, they are a top team in this division, and on the day it just wasn't to be.''
But at the conclusion of play a WRU presentation party - headed by Chairman David Pickering - duly handed over the silverware to delighted Neath captain Lee Beach.
Rees added, "Hopefully, more success this season is on the way, but just for the present we will enjoy tonight.''
Neath started the game badly, missing a crucial tackle on Roger Davies in the second minute, and the wing took his chance to touch down at the posts with fullback Leigh Halfpenny converting. Cardiff had flanker Jevon Groves yellow-carded on 12 minutes for killing the ball, and fly half Arwel Thomas duly converted the penalty.
Just three minutes later Neath hooker Jimmy Richards had a try ruled out but scrum pressure and forward momentum eventually told and the Blacks won a penalty try, converted by Thomas. Groves returned in the 23rd minute, and Cardiff levelled on 10-10 via a Halfpenny penalty. Neath went down to 14 men on the half-hour with flanker James Merriman binned, and Halfpenny punishing the visitors.
Back came the Blacks in the 32nd minute with slick hands from Arwel Thomas and captain Lee Beach who released fullback Gareth King over for an unconverted try. Cardiff wing Davies grabbed another try in the 35th minute to gain the lead. Halfpenny missed the conversion, but from an identical spot he nailed a third penalty, extending Cardiff's lead to 21-15 at the half time whistle.
The 62nd minute saw a high punt from Arwel Thomas put Halfpenny under pressure, which almost yielded a try for Wales U18 wing Kristian Phillips. Cardiff gained the next vital score in the 74th minute via a fourth Halfpenny penalty. Neath had a try disallowed from wing Matthew Nuthall in the 77th minute for a foot in touch, and time wasn't on the side of the visitors.
Three minutes into injury time, captain Beach picked up from a five-metre scrum, and replacement scrum half Roberts nipped over for a consolation try converted by replacement fly half Howard Thomas.