Some of the cheaper fancy blue diamonds in the diamond market today are man made or treated blue diamonds. They are still fancy diamonds and still are very precious and their price can be high depending on carat size, cut and quality.
The technology that it takes to grow an fancy diamond or color a diamond is not a cheap one. The diamond is still authentic and %100 real, only it has been treated or grown in a lab. Still, finding a natural Blue diamond in nature costs a lot more then growing one in a lab or treating a regular diamond to color it fancy Blue. This is why the cost of a treated or man made Blue diamond will usually be %70 cheaper than a natural one.
Diamond mining companies will usually mine and process tens of thousands of tons of ore per day just to find one carat of natural colored diamond. This is why the treating or growing process, makes Blue color diamonds much cheaper to produce and buy.