Why does God ask Abraham to walk blameless? This question is an offshoot from this question and it’s comments.  The question is:   

Why does God remains silent for 13 long years after Hagar gives birth to Ishmael and then suddenly commands Abraham to walk blamelessly before Him. Was God annoyed with Abraham for some reasons?
There are several accounts in the scripture where we can find Abraham lacking in the uprightness God desires of His children. 

Although a man of faith as we know, there were times Abraham doubted the ability of God to provide, deliver and fulfil His promises. In Gen. 12, after God led him to the land of Cannan and famine struck, he didn't trust God for provision. This indeed is lack of faith although it was the early stage of his conversion. Hence, Abraham travelled Egypt to be confronted with a circumstance that made him engage in conspiracy to deceive the Egyptians (vs 12). Although many believers have called Abraham a liar and all sorts, words which I'm very careful to use because the scripture didn't explicitly say so. I rather stick to the point that Abram doubted God and deceived Egyptians. Thereafter, the doubt of Abraham and Sarah took a different dimension resulting in the extra- marital union with Hagar which preceded the birth of the troublesome Ishmael. 

These are not blameless life God wants. Morever, a lot of other things that might not have been recorded in the scripture because they won't be necessary for our learning might had taken place necessitating God's rebuke. What this tells us is that Abraham was a man just like us and wrestled with challenges of life and sins as we do today. Another thing to learn is that God's standard has always remained the same: ' walk before me and be thou perfect'.