Quote Originally Posted by Jayabalard View Post

Not at all, lawns are explicitly called out as DC 15. Lawns have grass. Having grass does not make the check any easier.

The grass is blackened because of the passage of many orcs (something similar happens all the time on the local college campus, where the students make their own paths); it's simply trampled down. This does not help him spot Pippins print in any way.
Firm Ground

Most normal outdoor surfaces (such as lawns, fields, woods, and the like) or exceptionally soft or dirty indoor surfaces (thick rugs and very dirty or dusty floors). The creature might leave some traces (broken branches or tufts of hair), but it leaves only occasional or partial footprints.
It's explicitly mentioned that the reason it's a hard check is that you only have occasional or partial footprints.

for he had seen footprints that went that way, branching off from the others, the
marks of small unshod feet.
Not partial tracks or occasional footprints. The likely reason is that the long grass bent very easily. Long grass bends and breaks in the middle, and holds tracks for a long time. That also makes it easier to see where a few hobbits ran off.

It's long grass as Aragorn sang around p 4.

Through Rohan over fen and field where the long grass grows
The West Wind comes walking, and about the walls it goes.