Full load, what specifically do you mean? Normal cruise rpm OR did you push the throttle all the way for max. rpm [WOT]? If full rpm , WOT, then have you done that before? What were the results before? What rpm was reached.?
By suddenly do you mean BETWEEN this run and the last run OR do you mean while you were out on this run and all of a sudden they started black smoking?
Cold weather should have no effect except to increase air from cooler, denser air, which the engine should like. Diesels like all the air they can get unlike gas engines.
.
Black smoke can be caused by several problems. It is almost always either overload or lack of air.
OVERLOAD
-props incorrect
-props fouled or damaged
-fouled bottom
-too much added weight which sneaks up compared to the way the boat was when new[er].
Since this happened suddenly and to both engines [ is that correct???] then I doubt the props are wrong but they could have been fouled by a line or seaweed. Not big enough to stall the engines but it would goof up the normal water flow and add a lot of load. Try a diver and get the props checked for fouling.
LACK OF AIR
-air filters, turbo, aftercooler, exhaust system,
Have you looked in the engine compartment.? At the air filters? If they are dirty that will cause insufficient air for the engines and black smoke. If you have an exhaust leak that will allow soot to plug the filters and that plugging can be quite quick. Soot should be quite noticeable. Regardless, Try changing the filters. It may be that plugging , if that is the case, reached a point where you noticed especially if you ran the engine a bit harder than normal. If this is the case then eventually it would have made itself known even at lower rpm.
I suggest checking/cahnging the air filter first since they are relatively cheap to replace and should be little effort. Also the diver as he/she too will be relatively cheap.
Some of the other things are going to take some work to do and some, possibly serious, expense to find and sort out.
Report back what you check and do.