In general there is absolutely no problem wandering on any path or track in the Bulgarian countryside. The pastures, forests etc around most villages are traditionally used by the locals to graze livestock, gather mushrooms, herbs and berries etc, and no-one will mind at all if you explore the region on foot, as long as you avoid crops and of course lush grassy meadows which are going to be scythed. Access to nature reserves, hunting reserves etc has more restrictions, and these you should only enter along officially marked and designated hiking trails. The other types of region that have quite strict access restrictions are so-called "water catchment zones", special areas that are used as sources of drinking water for surrounding settlements. Most of these are strictly no-go areas, but they are usually signed and access tracks have barriers. Basicly if a path or track is clear and open you are likely to have no problems wandering along it.
Of course the interesting question is what will happen now more and more foreigners are buying properties and land in Bulgaria. Are "we" going to fence off "our" land, put up signs saying "private" and "no-entrance" on tracks and paths that run through them? Its a problem that has become all to common in some other parts of Europe with newcomers jealously defending their new little estates! Unfortunately it is already starting in BG. I am afraid I have already found that some of the lovely cliff-top and coastal walks I used to do are now inaccessible, and even in and around some mountain villages in the Rodopi, pastures over which I once freely roamed have now started to become fenced off and divided up into little parcels with signs appearing saying "private property". Are the new British land-lords going to bring with them their old "Enclosure Act" mentality"?