Starting autumn 2015 WotC will move away from their 3 block / 1 core release (explained below) to just 2 blocks twice a year.
“Expansion sets beginning with Mirage have come in groups (usually three) which form a block (the preferred term in actual gameplay) or cycle (the preferred term when describing the associated storyline) consisting of one large “stand-alone” expansion set of more than 300 cards, followed by one or two small expansion sets of less than 200 cards, which continue the themes introduced in the large set.”
They hope this will relieve some of the player concerns of a constant reinvention of the game. With this move that also hope that it will keep the the players excited about what they release and in turn keep the game fresh.
“ Since the beginning of the block structure (Mirage block, way back in 1996), the third set has always been a problem child. It needs to have enough cohesion to feel as if it belongs to the block, but it has to have enough differences that it manages to spark the interest of the players, who have already been playing the environment for seven or more months.”
“Who exactly is the core set for? If it’s for beginners, why do we only put it out during one point of the year? Is that the one time we expect new players to start? And why do we keep adding new cards and bringing back mechanics? The beginners don’t need any of that and it only leads to making the set more complicated.”
The new set labeled “Blood” will be release in autumn of 2015. You can read more here.