It is far better to pour one glass of borax into one gallon of water, stirring until the powder is very dissolved. It's a good possibility someone familiar will have gone through this before. Black mold poses an exceptionally high risk including extensive internal and external medical problems as well as pulmonary hemorrhaging in infants. It could possibly be extremely dangerous.
If the thing is what looks like a concern or smell a musty scent, then you would be a good idea to have your property inspected to generate sure.
