Boots became a symbol of power in the gay leather community. The old Guard community (pre-1990s) had strict protocols and roles and there wasn't much deviation from the codes they followed. They believed in discipline and respect, and their leathers and boots were important for their image. These items needed to be taken care of, and as many men were getting into BDSM they found a resource for the care of their leather and boots, at their feet.
A bootblack is one who, like a shoe shiner, cleans, polishes, and maintains the quality of leather boots. They are skilled in leather care and know just how to make even an old pair of boots glisten like new. To many it is an art form where the bootblack takes special care to do a perfect job and not miss any details. There are many methods and supplies that can be used. Many well-experienced bootblacks have their own ways of doing things and they are usually good people to get information from as they are experienced and know what works. It is a good idea to do research and possibly work with a bootblack before attempting to try it out on a new expensive pair of boots. If you are working with other people's boots this is especially important because not only are most boots expensive, people take pride in them and wouldn't want them to be ruined by an inexperienced bootblack.
There are specific polishes and waxes that one should and shouldn't use when boot blacking. A common mistake that people make comes from the term "spit-shine". Spit shining is where one uses their saliva and a cloth to polish a pair of shoes. While this might have been okay in the old days for regular shoes, it is not something you want to do with a pair of nice leather boots. Spit contains acids and properties that will damage the wax or polish on the boots, and it doesn't even work that well for shining.
In the case of BDSM play spit shining might be more acceptable. If you have boots that have already been shined, or boots that you don't mind getting saliva on, then you can have a lot of fun. In the Femdom world boot worship is one of the most common activities. Even those in the mainstream have heard and used phrases referring to licking one's boots. A sexy Domme in stiletto heels with a corset and a whip is the image that comes to mind when talking about Dominatrices. Boot licking is a way to humiliate the submissive and put them in their place, and they are more than willing to be put there. It can be a way to show appreciation for the Domme while still being denied access to her body.
As for the male dominant relationships there are many things one can do. Some people enjoy having to lick dirty boots, while others enjoy being kicked or pushed down by their Dom while he is in boots. Boots can be that special added touch to the perfect outfit or uniform the Dom is wearing, or they can be a way to make a submissive into a dirty bootlicker.
There are so many other activities that that would take pages to talk about such as boot lacing, crushing, and all of the difference scenes one could have with boot worship. For now I'll just leave you with a hint of what's out there. After all, exploration is part of the fun!