You are correct. I know the sign you mean at London City, it's at taxiway C. The ramp is immediately behind and aircraft in that position, which are just turning to face away from the ramp area, if they really opened up the throttles to move round the corner onto the runway, could blow some things over behind them. So it means "PLEASE USE THE MINIMUM THRUST YOU CAN TO MOVE FORWARD AT THIS POINT AS WE DON'T WANT THE RAMP PERSONNEL BLOWN OFF THEIR FEET BEHIND YOU". That's a bit of a mouthful so "CAUTION YOUR BLAST" is shorter. As you might imagine, it follows An Incident.

There are various other one-offs around the world. LAX's "NO TURNS BEFORE COAST" at the start of the runways always amuses me, too.