There is more than one answer to the question, "why does my Blonde hair turn Yellow over time?". In order to answer this, we may as well explain how we get to Blonde in the first place. If hair is dark or gray, we must remove natural pigment and tone. Depending on how dark, we may need to pre-lighten and tone to get the desired shade of blonde and avoid the Orange stage. Hair with no color left is still a bit yellow, provided we do not get into the melted stage. From there we add different levels of Blue, Red, and Yellow to get the desired effect. Over time these added colors fade and uncover what was left. Adding Violet to yellow hair can make it white or silver, which actually looks lighter than before. So when it fades it appears to be darker, but this is just a trick of how light is reflected. So, answer number one is... Your toner faded out.
Answer number two. Well, let's go with the explanation again. When we lighten, we remove the inside parts leaving space to be filled. Nature does not like things that are not balanced. So, if you have a high mineral load in your water, the color of that mineral will penetrate and stay. The color of certain chemicals will penetrate. Cigarette smoke leaves a yellow residue. Chlorine is green. Most blondes can remember swimming and having a green tint. So, answer number two is environmental contaminates that penetrate or stain. Light hair shows stains because it is light.