How will the taste of black tea change when I infuse it with water colder than 90-100 degrees Celsius?
TFD's answer is true: the most likely change is the reduction in some astringent flavors.

I would just add that people have been brewing black tea for iced tea for many years at or near room temperature (e.g., the traditional "sun tea" method), or even in the refrigerator ("cold-brewed" tea).  The flavor notes which come out are different, but a lot of people still find the tea produced to be acceptable or even preferable to "hot-brewed" tea.

Granted, these processes are used mostly for tea meant to be consumed at cooler temperatures.  But my point is that those who insist that black tea must be brewed as close to 100C as possible are just brewing one possible "version" of tea from those leaves.  In my own experiments, I've found that certain black teas taste good when brewed at lower temperatures for a long time, though others end up "unbalanced" in some way.  (Sometimes, they simply taste "weaker" for some reason, even when still brewed longer to take into account the lower temperature.  Sometimes this can be fixed by adding more tea leaves per cup; other times this still creates an imbalance.)

I could cite a number of studies which show the various amounts of certain chemical components in tea and how fast they dissolve at various temperatures.  But those really won't take into account the balance of individual flavor components in a particular black tea.  In this case, I think it's really a matter of personal judgment to see whether your particular tea tastes fine to you brewed at a lower temperature.