It sounds to me like you got a batch of really unstable gasoline. Some of the trace compounds in gasoline react with oxygen in the air to form gum (polymers). Normally, the rate of gum formation is slow either due to a low level of these gum forming molecules or due to the presence of stabilizers added to the gasoline. It sounds to me like the gasoline you got had a very high potential gum and quickly oxidized. This would account for the dark color as well as the fouled injectors