"A life as long as a darkroom" means that life can be as long as a long darkroom, which may refer to all kinds of challenges, sufferings or difficulties that people experience in life. But "it doesn't prevent me from writing clear spring poems" means that I still have the ability to express beauty, light and hope despite many difficulties. This sentence expresses the author's firm belief in creativity and imagination, and shows a positive attitude when facing challenges.
Generally speaking, this sentence conveys a positive and hopeful view, that is, no matter how dark and difficult it is, one's inner strength and creativity can still stand out and bring beautiful and bright things.