A black color in sedimentary rocks almost always indicates the presence of organic materials. Just one or two percent organic materials can impart a dark gray or black color to the rock. In addition, this black color almost always implies that the shale formed from sediment deposited in an oxygen-deficient environment.
http://geology.com/rocks/shale.shtml
Mudrocks have low porosity, they are impermeable, and often, if the mudrock is not black shale, it remains useful as a seal to petroleum and natural gas reservoirs. In the case of petroleum found in a reservoir, the rock surrounding the petroleum is not the source rock, whereas black shale is a source rock.
https://en.wikipedia.org/wiki/Mudrock
Although drilling can extract large amounts of oil and natural gas from the reservoir rock, much of it remains trapped within the shale. This oil and gas is very difficult to remove because it is trapped within tiny pore spaces or adsorbed onto clay mineral particles that make-up the shale.
http://geology.com/rocks/shale.shtml
These other constituents in the rock are often determined by the shale's environment of deposition and often determine the color of the rock. Like most rocks, the color of shale is often determined by the presence of specific materials in minor amounts.
http://geology.com/rocks/shale.shtml
Shale is a fine-grained sedimentary rock that forms from the compaction of silt and clay-size mineral particles that we commonly call mud.
http://geology.com/rocks/shale.shtml
In order for a shale to be a black shale, it must contain more than one percent organic carbon. A good source rock for hydrocarbons can contain up to twenty percent organic carbon. Generally, black shale receives its influx of carbon from algae, which decays and forms an ooze known as sapropel.
https://en.wikipedia.org/wiki/Mudrock
The term mudrock allows for further subdivisions of siltstone, claystone, mudstone, and shale. For example, a siltstone would be made of more than 50-percent grains that equate to 1/16 - 1/256 of a millimeter. Shale denotes fissility, which implies an ability to part easily or break parallel to stratification.
https://en.wikipedia.org/wiki/Mudrock
Mudrocks make up fifty percent of the sedimentary rocks in the geologic record, and are easily the most widespread deposits on Earth. Fine sediment is the most abundant product of erosion, and these sediments contribute to the overall omnipresence of mudrocks.
https://en.wikipedia.org/wiki/Mudrock
Black organic shales are the source rock for many of the world's most important oil and natural gas deposits. These black shales obtain their black color from tiny particles of organic matter that were deposited with the mud from which the shale formed.
http://geology.com/rocks/shale.shtml
Mudrocks, especially black shale, are the source and containers of precious petroleum sources throughout the world. Since mudrocks and organic material require quiet water conditions for deposition, mudrocks are the most likely resource for petroleum.
https://en.wikipedia.org/wiki/Mudrock
which environment would be likely to produce a black shale?
An oxygen-deficient environment would likely to produce a black shale. Oxygen-deficient environment would be likely to produce a black shale.