How Did Layne Staley Become the Singer of Alice in Chains?

Why is Layne Staley not credited for vocals on the post-mortem Alice in Chains album 'Black Gives Way To Blue', while I can clearly hear his vocals on at least two songs?

Mother of Layne Staley raising money for addiction resources

Consequence - Layne Staley, the original singer of Alice in Chains, passed away 20 years ago today. R.I.P. 📸: John Atashian/Getty Images

Layne Staley: The Tragic Death of Alice in Chains Lead Singer

Alice in Chains' Layne Staley: Needle and Damage Done

Alice In Chains Announce New Layne Staley Release – Punk-Rocker

Jerry Cantrell - Wikipedia

How Jerry Cantrell Tricked Layne Staley Into Joining Alice in Chains

Metal Hammer - On this day in 2002 Alice in Chains

Is Layne Staley the best rock / grunge vocalist of 90's era? - Quora

Eight Previously Unreleased Tracks With Late Alice In Chains Singer Layne Staley Reportedly Exist

Dirt by Alice in Chains - by Craig Lyndall