I have seen mushrooms like these growing in acidic much under pine trees and they make that Black ink substance, I wouldn't worry though, I had a fungus called Tapioca Slime, came in because of the mulch layer in the garden, so dont panic, instead try referring to a field guide if you can, or simply study through observation.
Like Sepp Holzer says: we should try reading the book of nature.
It looks like the remnants of a Coprinopsis_atramentaria They often pop up in straw mulch after the rain, but there are lots of sub species and some stick around longer than others.
I've never found them to be a problem in my garden spaces. It' seems to me that they are just turning straw into plant food.
I agree with Craig. Looks like a variety of ink cap mushroom. That inky mess is actually a spore slurry, which the fungus uses for reproduction. They're totally harmless; you can even eat certain varieties before they turn inky. Like Craig said, they're just helping break down organic matter in the garden.
"There is nothing, Sir, too little for so little a creature as man. It is by studying little things that we attain the great art of having as little misery and as much happiness as possible." - Samuel Johnson