Dark blood under big toe nail, any idea/concern?

Depends on the cause. If you injured the toe it will generally grow out in time. If there was no injury you should have it examined to be sure it is not a lesion under the nail which would need attention.
Subungual . Hematoma is name and if fresh then can be drained. If not then will grow out with nail unless major damage to nail matrix. Sometimes trauma will make it fall off.