Hosts Tumble took the lead at Parc Y Mynydd Mawr through scrum-half Rhodri Jones before Blackwood hit back twice through Gary Wilkes and Josh Law.
Magpies wing Josh Davies skipped over in the second half to level the scores 12-12 and send the game into extra-time but Blackwood showed the form that had brought six successive wins and Pritchard held his nerve to slot the winning kick.
It was just as close at Brynmawr who squeezed past visitors Bala 21-20 at the Rec while Llanishen edged past Tenby United four-three on try count, after the sides finished level 22-22 of a closely-contested tie in Cardiff.
A hat-trick from Jordan Rees sent Rhydyfelin into the third round in a comprehensive 63-7 rout away to Carmarthen Athletic.
The Division One East leaders were dominant from start to finish as they swept aside their opponents from Division One West with a brace from Ross Lucas and Rhys Phillips, Luke Rees, Dai Gratton and Scott Pritchard all grabbing a try. The visitors had led from the first minute when Gratton slotted over an early penalty and the result never remotely looked in doubt.
Mold crept past Bridgend Athletic 18-17 in a tight encounter at Chester Road. Tries from Ryan Kirby and James Grundy plus eight points from the boot of Cai Edwards were enough to put the Flintshire outfit through to the next round.
Last season’s beaten finalists Penallta had too much quality for Brecon as they ran out 41-6 winners at Ystrad Fawr. Braces for Lewis Barnett and Elliot Keep were the highlights as the Pitmen ran in seven tries with Brecon only able to muster two penalties in reply.
A try by Gethin Gibby and five points from the boot of Rhys Davies were not enough for Crymych to progress against Abergavenny, who won 13-10 at Bailey Park. Cardiff Quins edged a thriller, 35-32, with Ynysybwl.
In the National Bowl, reigning champions Burry Port kept their cup defence going with a 32-10 win over St Clears. Two tries from hooker Iwan Roberts set Burry Port on their way at The Park with Richard Parker, Llyr Simms and Luke Randall all following. Ten points from the boot of Ben Williams also kept alive their dreams of consecutive Bowl wins.
Denbigh also secured their place in the third round with a 27-18 win over Crumlin while Trimsaran won a helter-skelter match 23-19 away to Menai Bridge.
The hosts had led 5-0 at half-time only for Trimsaran to score 20 unanswered points after the break. The Angelsey outfit brought the score back to within a point only for a late Trimsaran penalty to halt the comeback.
Monmouth were convincing 62-7 winners at home to Cwmcarn United, while Cambrian Welfare kept hosts Tonmawr off the scoresheet in a resounding 40-0 victory.
