Why do the seam an agent can see black seam?

Why do the seam an agent can see black seam?

a: recently there are many new beauty consulting seam craftsmen seam an agent manufacturers, after doing perfect seam, why ceramic tile edge will leave a black seam? Actually the main reason is on both sides of ceramic tile aperture not clean, such as cement magazine, lead to make it perfect seam 2 way black border will occur. So do the seam is later, must put the ceramic tile edge clean, with a pen knife, shovel knife, crack-cleaning cone tools such as clean up.