苍穹 is poetic and literary - what you find in classical poetry, sci-fi titles, and lofty speeches. Everyday speech just says 天空 ('sky'). Reach for 苍穹 when you want gravitas.
memory
苍 = 'deep blue / ash-grey' (think 苍天 'the heavens') + 穹 = 'arched / vaulted' (depicting a curved roof). Together: the deep-blue dome overhead.
