I will be shooting a family portrait where the husband has very dark hair. I was planning on using my black muslin background with illford Delta 100 B&w film and my 550 EX speedlite (bounced). Is the black background not a good idea? Any suggestions are welcome.
Another color of bg would probably be better, but if you had to go with the black bg, just make sure your subject is far enough in front of it to get seperation. You could also consider, if at all possible, using a hair light. Another option might be to use your light at more than a 45degree angle (ie. closer to 90degrees) to provide texture, of course this type of texture lighting doesn't make for very flattering female portraits though.
Shining an additional light at the background will help separate the subject from the background when they tend to blend in. I did this with some studio shots I took of my daughter a month or so ago. Her hair is almost jet black, and when I used the black background, I made sure there was a light on it to separate her from the background. Make sure the subject is at least 6 feet in front of the background, too.
I also used Delta 100 and had very nice results. I love that film!
I would go with a different background, medium blue shows up as zone V grey on b&w film. The problem with solid black is that you may find it difficult to set the hair light for the one person whilst not having the shadow fall on the other person. With one or two people in the photo this is more controlable, but with 3 or more, nothing but a hassle.
Instead of lighting the black background (getting enough light to make separation from the subject is a potential problem), try rim lighting the subjects to achieve separation.
In choosing a backgound, you may wish to see how the subjects are dressed; IMHO, people in dark clothes do not look well on a light background and visa/versa.
I think Ron gave you some good advice with the blue background, it will be easier that way. When I started school, I was given the idea to photograph an old kodak color chart using b&w film to help distinguish between gray zones for portrait set-ups. Works like a charm!
Have fun and shoot it with the black back drop. Take your reading on the black and bracket one stop. You will suprised how great it looks. It is kind of like opening a couple of stops with snow. You should get good facial tones and hair highlights without completly blowing out the highlights.
Pros often shoot against dark backdrops. This technique really helps present the subject as the center of attention and tends to make colors pop and improves the appearance of skin tones. However, you must separate the hair (or head) from the backdrop. That's done with a hair light, which is normally placed above and behind the subject, and sometimes a little bit to one side. The hair light reflects from the hair to add highlights and texture. This is doubly important for subjects with dark hair, and prevents the hair from turning into a dark, detail-less blob in the image.
With your flash setup, that isn't going to be possible so I'd strongly recommend against using a dark background in this case.