If my gums don't hurt but bleed for a short time when I floss my teeth, is this always a sign of gum disease, or is it merely a sign that my gums aren't yet toughened up by frequent flossing?

Gingivitis is the first visible step on the progression to http://www.mayoclinic.org/diseases-conditions/periodontitis/basics/symptoms/con-20021679. Your gums don't "toughen up", they either have the minor infection attendant with gingivitis or they don't. As with many things, lapses in self-care you could get away with 5+ years ago tend to hit you harder as you get older and periodontal health is among them. Alternately, it is possible to floss far too aggressively causing bleeding by itself but in either case, you should see a dentist.

Bleeding when you floss is an indication of gingivitis, the gum infection that can lead to periodontal disease, the loss of the tissues that support the teeth. Periodontal disease is generally painless, and can take many years before detectable by the sufferer alone. At each dental exam, or dental hygiene visit, your gingiva should be examined and measurements should be taken to assess the health of the tissue. The measurements, along with the presence or absence of bleeding during the exam are essential in assessing evaluating oral health. Gums are not toughened by flossing. However, assuming you do not have periodontal disease, if you continue to floss consistently you will see the bleeding reduce and then stop completely as the mucosal cells lining the tissue that surrounds the teeth are replaced, a process that takes 7-10 days. In the presence of plaque bacteria these cells become ulcerated. When stimulated these ulcerated cells bleed. As you continue to routinely disrupt the plaque bacteria in these areas the bacterial environment changes to favor less damaging bacterial species. In this improved environment cells do not ulcerate, and it is as these new cell develop that you will see the bleeding subside. Gingivitis is an infection that requires home care to control and reverse.  It's terrific that you have begun to clean between your teeth.  If you find that using traditional floss isn't a habit that sticks, try an alternative. Research has shown that there are ways to clean in between the teeth that are equally effective. Interproximal brushes, sometimes called proxy brushes, have been show to be very effective, and people often find them easier to use. It is important to be sure that you use a proxy brush that safely fits the spaces between your teeth without putting too much pressure on your gums, but there are many shapes and sizes. There are even options such as Soft Picks that are able to access most spaces safely and easily. Another option is a device that holds the floss for you. There are disposable and durable options available. A new device that seems to be very well received is Gum Chucks, made by OralWise. They have a really fun line for kids that includes cool characters and an entire backstory. They also offer a line for adults, and one specifically for those in braces that makes flossing significantly easier and far less time consuming. Again, it's great that you are taking up interproximal hygiene.  Your oral health and systemic health will thank you. Hope this helps.

My gums bleed if I brush too hard, and they bleed every time I floss. I've never had any cavities etc, and the dentist can see absolutely no evidence of gum disease. My skin is also prone to blistering - I seem to have delicate skin. Do you blister easily too, such as when you walk a long way or wear new shoes?  (Or in my case, peel a few potatoes - seriously, blisters from holding a potato peeler!) Perhaps get a dentist to check your gums, and your flossing technique. I've just come to accept that my delicate skin won't toughen up, so it's pink spit for me.