Though the black spots may be on the membrane filter of Simple Tester, it has no problem on using this tester.

This black spot can be generated during production,

(1) Dyeing spot generated during dyeing the membrane filter.
(2) Color matter spot generated during connecting the absorbing pad and membrane filter.

So it has no effect on microbe inspection with Simple Tester. We choose to use the color matter with a minimum effect on the growth of germ.