The Blackhearts main role was counterterrorism and counterintelligence, so probably not much need for a 'Mech in their day to day job. However, like real world spec ops, many of the Blackhearts would no doubt have been cross trained, plus would retain the skills of whichever branch of the military that they'd originally belonged, so at least some of them should have had no problem strapping on a 'Mech if needed.
The SLDF's elite special ops 'Mech unit was the Foul Weather Fiends.