import re
text = "1538 a\n123\nskua456\n789\n5\ng\n15563 blu55g\nb\n456\n16453 a\n789\n5\n16524 blu\ng\n55\n1734 a\n987\n987\n55\naasf\n552\n18278 blu\nttry"
print( re.findall(r'(?m)^.*a(?:\n.*)*?\n.*blu.*', text) )
print( re.findall(r'(?m)^.*a(?s:.*?)blu.*', text) )
aW1wb3J0IHJlCnRleHQgPSAiMTUzOCBhXG4xMjNcbnNrdWE0NTZcbjc4OVxuNVxuZ1xuMTU1NjMgYmx1NTVnXG5iXG40NTZcbjE2NDUzIGFcbjc4OVxuNVxuMTY1MjQgYmx1XG5nXG41NVxuMTczNCBhXG45ODdcbjk4N1xuNTVcbmFhc2ZcbjU1MlxuMTgyNzggYmx1XG50dHJ5IgpwcmludCggcmUuZmluZGFsbChyJyg/bSleLiphKD86XG4uKikqP1xuLipibHUuKicsIHRleHQpICkKcHJpbnQoIHJlLmZpbmRhbGwocicoP20pXi4qYSg/czouKj8pYmx1LionLCB0ZXh0KSAp