Did you try to put double quotes ?
set "java_opts=%java_opts% -Dblah.blah.blah=this+is+a+sting(just+one+value)+to+be+added"
Hope this helps
Yes I tried that but the error is the same, it still seems to break the string at the ")"
Maybe you can have a look at this page
He suggests to try one of those
Escaping - The caret(^) is the escape character, using one before the character(including that caret itself) escapes it. When delayed expansion is enabled two carets are needed for exclamation marks, see %variable% !catches!.
Doubling - Normally used for percentage signs, as the name suggests you simple input two where you want one.
Quoting - Where possible, like filenames, surround the string in double quotes.