The latest Cateye rear lights now come with their "flex-tight" bracket - a ratcheted plastic band that you tighten by hand. See
Cateye LD270 on Wiggle
for instance. I've had some success attaching this to the thin tubing of a rear carrier, though I had to pad it out with a padding strip from an old bike computer mounting - you could wind insulating tape around a few times for the same effect.
The Blackburn Mars 2.0 definitely would NOT fit without modification. It comes with two mounting options, one suitable for a seatpost and one for a belt clip. Neither is easily adaptable to a rear carrier.
Another alternative that will suit many rear lights (but not the Mars 2.0) is to buy a
mounting bracket - one end bolts to the carrier, you screw the light's normal bracket to the other end. Annoyingly this will only fit round a single tube's width, so it won't attach to the most obvious bit at the back of a Blackburn rack where two tubes are welded together in parallel. You have to fit it to one side or the other, but it's easy to arrange it so that the light itself sits more or less centrally. I use this for the Cateye TL-AU100BS and similar. It's very solid.