By Nina Chestney LONDON (Reuters) – President Vladimir Putin signed a decree on Thursday requiring foreign buyers to pay roubles for Russian gas from April 1 or see their contracts halted, a move described as “blackmail” by European states. WHAT IS BEHIND THE CHANGE? The move is in retaliaton against Western sanctions imposed on Russia over its invasion of Ukraine. Moscow, which says it is conducting a “special military operation” in Ukraine, describes the Western measures as an economic war. Getting Western countries to pay in roubles would blunt the impact of their curbs on Russia’s foreign …