GREP styles are defined inside a paragraph (or paragraph style). Which means that you’d first have to apply a paragraph style to those titles — in which case you don’t need a GREP style, you simply make the formatting part of the paragraph style. Also, Anne-Marie correctly wrote that you can look for \n, but that’s a forced line break. If those line breaks are paragraph breaks then you can’t use a GREP style.
Why don’t you apply a paragraph style with a GREP find/change? Look for (?< =\r\r).+ and apply a paragraph style. And if you set space before in the paragraph style, you can remove those two line breaks (which you really should do):
Find what: \r\r(.)
Change to: $1