What is a visa-free stay?
Citizens of certain states (e.g. the USA, Canada, Brazil, Australia, New Zealand, Japan and others) may reside in Bulgaria without visa for up to 90 days within a period of 6 months, using only their international passport. Type “C” visa is not required. Check HERE whether your country of origin is on the list of states where visa-free regime applies.
Are there any privileges for holders of Schengen visas?
Since 2012 Bulgaria unilaterally applies visa-free regime for holders of valid Schengen visas. If you are a holder of such document, you may enter and stay on the territory of the Republic of Bulgaria for a period of up to 90 days, as of date of first entry, solely on the grounds of your Schengen visa without obtaining a Bulgarian document.
The opposite, however, is not valid: Bulgarian visa does not grant you the right to the Schengen area, meaning that you cannot reside in Schengen area using your Bulgarian visa only.
Be advised as well that if you want to work and live in Bulgaria you should obtain a long-term Visa – type D (see HERE).