There's the makings of a story here, and I would like to see "beyond" the blackness, however you may want to clean the opening up a bit. Contract some of your lines, dismiss others, thereby leaving room to add more of the story.
Nits: (1) no comma necessary between "high, grassy ground" or "fingers, just to make" or "long, sharp blade"
(2) "...and on the other side by black nothingness" would read better without the word "side": "...and on the other by black nothingness". Use of Tolkien's wonderful "further" might work here. On the further bank... [I love that word further, it's one of my favorites, it displays so much to a reader, but I go off topic, sorry.]
(3) Second sentence needs to be completely reworked, for in the first line you tell us the other side of the river was nothingness, yet here you say the far bank looked just like the one Cheyne was standing on. Yes you quantify this my stating "for about two paces", but still I read it as awkward and misinforming. If both banks resemble each other near the river's edge then perhaps state that earlier then introduce the nothingness.
(4) "leaving nothing but a black as dark as an evil person's heart." I'm having difficulty getting beyond this point. For one, "a blackness" might read better than "a black", but that's if you want to keep the phraseology. I, for one, don't care for the metaphor you chose to use here, but that's me.
(5) You mention Cheyne is some distance from the "blackness" (Even from this distance...), then you go on to write that his fingers go numb when he puts his hand into it.
Now I do like the part where you write, "...it all just stopped, like a long, sharp blade had simply cut away the earth and trees" and actually I would stop there and not include the "...and peeled them away" part. Your last sentence runs on and can probably be broken down into two somewhere in the vicinity of your semi-colon.
Good job, though. Keep writing.