The Borders rallied with scrum half Chris Cussiter, back from the shoulder injury he suffered last November, snipping around the fringes to prove his fitness ahead of Scotland's RBS Six Nations opener at Twickenham next Saturday.
After Borders fullback Calum MacRae earned himself a yellow card for a high tackle on flanker Jamie Ringer, the Dragons ran riot with three converted tries in seven minutes soon after the break.
First Black grabbed his second from close range, then No.8 Nic Fitisemanu smashed through the defence moments later to wrap up the bonus point. Scrum half Alex Walker dived in under the posts to leave the Dragons cruising at 34-0 and compound the Borders' misery.
With time running out, fullback Gareth Wyatt and replacement wing Richard Fussell ran in a long range converted try with Thomas bagging five conversions on the night.