The focus is on the fact that the walls are now blue - something the hearer doesn't know about. The sentence has to appear in some context. Of course you can say, "I painted my walls blue."
A: What did you do yesterday? (A specific time has been mentioned - past tense)
B: I painted my walls blue.
A: Why is your wife upset?
B: I've painted the walls blue. (The focus is on the walls being blue now, which the wife apparently doesn't like.)
A: What have you done lately?
B: Well, I've painted my walls blue... (Answer is in the same tense as the question - parallelism)