Want to know the reasons for water in the new borewell turning blackish

Dear Sir,

Last summer I dug a new borewell which is yielding good amount of water. Recently we have noticed a change in the colour of water which has turned blackish. The mud in this area is slight whitish as we seen in the farms hence we are not sure if its because of silt.

My questions are:

- Is this blackish colour normal ?

- How do we make sure that this water is safe for drinking?