yes you can, you don´t have to know everything to administer a website, but you can´t call them "guru"
I think you have to maintain a server to be a "webmaster" - if you just have a blog or a 1 click setup - you are just in data entry
I have a blog with a one click setup (most of my sites, in fact), but I claim that title based on the fact that I install custom scripts and tweak out (or create) my templates with css, xhtml and php. Fortunately, I've never needed to configure Apache, or anything that involved. I mean, I wouldn't advertise my services as such, but for my own purposes, webmaster is certainly the function I fulfill. OTOH, sometimes I go by Editor.