[waynelau]

Why the resulting solution after titration gradually turns blue "again" ?

[lavoisier]

You should give more details about how you proceed.

However, if the 'blue' you're talking about is the colour of the iodine-starch complex, you should think of what substances you have in solution when the stuff if colourless and what when it's blue.
The next step is: how can the mixture of colourless substances turn blue? What is the fundamental reaction involved?

If you can answer this question, you only need to figure out that your solution is not isolated from the environment, and therefore there are additional substances that can interact with it.