блатнячок - diminutive from блятняк which is colloquial form of блатной which is prison slang for "a professional criminal" (means that it is his main occupation in life). High hierarchy in Russian criminal system.
блатной - Wiktionary

many dictionaries translate блатной as a thief. That is incorrect. This confusion comes from a word вор which means "a thief" but on a prison slang the same word means "a high-ranked professional criminal"