What does Montague mean when he quotes Black and portentous must this humor prove unless good counsel may the cause remove?

The easiest way I've found to go about understanding Shakespeare is to first underline any Words in the text which you are unfamiliar with, and look them up. Make a list and write down their meanings so you can refer back to it at any time. When studying literature (especially Shakespeare), your dictionary is your best friend!

In this case, your Word list might look something like this:

"portentous" is a synonym for ominous, foretelling of dark days ahead etc

"humor" here is another way of saying mood (Romeo's being a world class "emo")

"good counsel" is exactly what it says it is - the help and advice of a caring friend

So what Montague is saying is that unless Benvolio can shake Romeo out of his depression, there's trouble ahead for them all.