Bottom (blue ) part of the flame is where the hydrogen is burnt. Once the methane is stripped of hydrogen what is left goes up as soot and burns too - but most of the oxygen was already used, so this part of the flame has much lower temperature and the color of the flame reflects this fact.
If the methane is mixed with oxygen it can burn much better, as carbon is burnt in lower parts of the flame, together with hydrogen. That's how Bunsen burner works.
The problem is, the blue part of the flame is relatively dark - not as bright as the yellow flame. Thus blue flame will be barely visible, while any yellow part will catch attention.