RSS readers such as Feedly are not really meant to for reading older posts, but rather to keep track of when new things show up.
That said, I personally would use Google reader (back before it was cancelled) to keep track of what I had read or not from any new blog. So there definitely exists RSS readers which allow the behavior that you ask for. Can't tell you one by name unfortunately.
(I use Sparse RSS, but it has the same problem as feedly and only shows the latest 10 posts when I add a new blog. That one also only works on the phone and doesn't sync to anything on the computer, so is only useful if that is ok for you.)