I notice that, when I have the A/C blower on full blast, my voltmeter reads around 11.5 volts. It comes up progressively when the fan speed is reduced, and does not seem to be affected by any other electrical load.

Is this a sign that the blower motor is failing?

Try putting a voltmeter on the battery. If it doesn't drop by a significant value when you turn the fan on then you don't have a charging problem.

The voltmeter is connected to the "supply line" under the dash. When the load on this line increases (Eg. Fan, Demister) then the resistance increases and if the fuses have added resistance through the fuses then the voltage will decrease more than it should.

When you turn your headlights on then the volt meter shouldn't drop as much since the source power is under the bonnet since relays are used.

It could be that the increased current used by the motor at high speed is flowing through a poor connection somewhere and this causes voltage losses at those connections. Other possibilities are a weak battery and/or low alternator output. Also, if the blower motor has dragging bearings and the motor is running slower than normal, the motor will draw greater current due to something called a low reverse emf. This is why a stalled electric motor blows fuses. I would check the blower motor circuit and clean all contacts. Be sure the battery cables are clean and tight. Check the alternator output as well. Only if all the rest checks out and if the motor is running slow, would I worry about the blower motor.

"...a sign that the blower motor is failing?"

Probably a sign that one or several fuses have contact resistance at the clips. Time for a "fuse box tune up."

Or possibly the brushes are just about worn out?

"...possibly the brushes are just about worn out?"

Umm, I don't think this wouldn't cause a voltage drop. A voltage drop is caused by increased resistance in the circuit or increased current flow through the circuit.

Worn brushes inside the motor would increase the motor resistance, which would decrease the current flow through the motor. This would mean the fuse circuit would experience less voltage drop, so the voltmeter would dip less when the fan was used.

A shorted motor would draw more current, and this would cause the voltmeter to drop significantly. I don't think these motors fail that way, however.

The Volvo voltmeter is connected in the fuse block assembly on the "down stream" side of a fuse, along with the various loads such as the blower. Any contact resistance at a fuse (corrosion) results in voltage drop at the point of resistance. This lowers the measured voltage.

This is all Ohm's Law.

You are absolutly right! The man knows his stuff.