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"
f = False
result = []
block = []
for line in text.splitlines():
	if 'a' in line:
		f = True
	if f:
		block.append(line)
	if 'blu' in line and f:
		f = False
		result.append("\n".join(block))
		block = []

print(result)