Can a fuse appear to be good but in actual fact be blown?

Yes

I have seen several fuse that appear ok, and in fact are defective.

The best way to test, is with a computer safe test light, there not cheap but if you want to get serious about auto electric you need one.

To test fuses, you are generally safe with the cheap kind.

1) Plug in or clip on your test light. 2) Test your test light against a known good source for voltage. 3) Turn on the ignition switch. 4) Probe both sides of your fuse, the results should be the same. Note that one or two may be cold on both sides (or negative if you have a computer safe tester) They generally power the interior lights. 5) If you find a fuse that is cold on one side and hot on the other, you have found the monster. 6) Replace it with the same type and size.

IF all tests well and you have glass fuses, test the metal clip on both sides that it clips into. They may require cleaning, and careful bending from water/ rust damage. If you have plug in fuses, check the integrity of the fuse holder.

I have seen all types and sizes of fuse, especially glass, that appear good, but are blown.

Relentless

Best way to check a fuse is take it out of the circuit and test its resistance. A good fuse will have very low resistance (less then 1 ohm). A bad fuse will measure "Overload" on your meter. You can get a cheap meter from and local hardware store for ten bucks. Even bring the fuse with you. The guy at the store will probably check it for free for you. Good luck.