hi,
black mark is the impression of paint applied on the glass bed. i am using black mat paint on frosted glass for best adhesion. i was using 30% support erlier, but it takes more time print and very difficult to remove. yesterday i did two trial with different bed temprature and got good result on lower bed temprature 40 degree rather than 50 degree. so it may be this reason. i have also tried same product with higher supoort angle, but got the same results. anyway thanks your reply.