Woah! What did you use in the middle and where can I get some?
It looks that way because it's been given a coating of flat black to hide some of the markings printed on the plastic. If you take a look at the left boot, there is still a white line that sort of shows through right down the middle.I've looked into products similar but yours appears to have the look of a wide ribbed rubber almost. Thanks for the help! I'll look into it.
I will add that there are some locations, rarely anymore, that sell the corrugated 4" drain tile in 10' lengths.