There is this for the built in sub-classes:
• https://help.blocsapp.com/subclass-reference-guide/
• https://help.blocsapp.com/the-basics-of-using-sub-classes/
In the above link, the sub-class for “Logo Link Image” is actually = “navbar-brand img” when used in Blocs.
You can also find references for Bootstrap 3 classes in general, here is one such example: Bootstrap All CSS Classes