EDIT: Oh by the way, I'm using my own template, so I guess I just need to know the names of the controls/variables to put in the template
I think the best way of seeing how the blog works is to look through the templates that come with the blog. Not much module specific information on blog but if you look through the BlogHolder.ss and BlogPage.ss files in blog/templates you can see what variables it uses. The latest posts, clouds are a bit more tricky since these are widgets. Each widget has its own template and controls.
What you need to know it that the 2 template folders are both part of the same theme.. this means that because of the naming SilverStripe (SS) reads them as related... so if you installed for example the forum module, you would create a blackcandy_forum folder to contain the templates for the forum and have that folder as part of your blackcandy theme.. if you create your own theme you will need to have folders like: YourTheme, YourTheme_blog, YourThem_forum etc...
One other note is that SS also looks in the module folder itself for templates... these are the templates you override if you create folders eg BlackCandy_blog (the WidgetHolder.ss file you found here simply overrides the one found in the blog module folder I believe).
Read the page I posted and take special attention to the section Overriding.