Ducts usually lie on top of the ceiling joists, leaving a 3.5" space between the duct and the ceiling. If for some reason yours is laying right on the plaster/drywall you can cut it off and replace it with a section of flex duct.

Is this new construction or remodel?

Being unaware of flexible duct identifies you as inexperienced. Not meant as an insult. My suggestion is to contact an HVAC contractor to look at your situation as the next step.
In building a house, the sequence is that the plumber gets first choice of the spaces for utilities-- the DWV system must have suitable fall-- because sewage does not flow uphill. The HVAC gets next dibs on the remaining space. The electrician gets whatever, if any, space that remains after that.
The present challenge that you face is not unusual for electricians. Can lights in a large room and down a hallway can be extremely challenging to get in a straight row because of framing and other utilities.

If its a ceiling fixture you cannot connect it to the duct it's not rated to hold any weight and the light may fall. You will need a bracket to span the two joists, you will need to use a metal box to attach to the bracket, if the regular box is too deep you can use a pancake box. They do make an old work version that you put in the hole and turn it til it's tight it spreads as it turns.

I suspected as much about the duct idea but good to hear from someone that knows his stuff. I've put in dozens of chandelier hanger boxes over the years and will probably need to do so here as well.

problem solved. Opened up the ceiling and only had to bump in the vent about an inch to allow a bracket to span the joists and install a pancake box. As the duct is very near the end of a long run anyway, there is almost no effect on the air flow.

That was probably your best option. Electricians, plumbers, alarm and media installers do that and worse to ductwork all the time.

For future reference, they do make a surface mount electrical box for these cases - it's only just over half an inch deep so it can be mounted directly on a ceiling joist. In a retro application, you'd be cutting the drywall carefully away to expose it, repairing/installing a vapour box over it up in the ceiling.

The downside to these thin boxes is they don't have a lot of space for wires, so you wouldn't want to use it in an instance where you have a bunch of wires entering unless you had room in the fixture to allow them to fit inside, assuming that's permissible by code.