When should I switch the black ink type to Matte Black?
You should switch when printing on non-glossy paper, such as Epson Presentation Paper Matte or Epson Velvet Fine Art Paper. Matte Black (MK) ink produces better black ink density on these paper types than Photo Black (PK) ink. See the link below for a list of Matte Black ink and Photo Black ink media types.
You need to switch the black ink type only if you disabled automatic black ink switching. Because this process consumes some ink, you should avoid switching black ink more often than necessary and select a default black ink type for the type of media you print on most often.
Note: To disable or enable automatic black ink switching, press the Menu button and select Printer Setup > Auto Black Ink Change, then select Off or On.Related references
Black Ink Type Media List