How can I make this look more like a flame blade?

There is no easy way to do this, practice is all you need. If you study flames and focus on this blade you could achieve it in 3-4 days. That is assuming you draw at least 25 a day.

This is known as brute force learning, the idea is that you take something you don't know and fail at making it around a 100 times. Artist use this when ever they get stuck with a single topic, like drawing a nose or eye, then just keep trying for a long time.

The important part is that you keep making the full sword, even if it is low quality, don't start a new one till it is done.

To make things easier you could take a image of a flame and a sword then place the flame over the sword blade. Use this as a background to draw over to get the sword you want.

Also this is a good set of "how to" tutorials on pixel art: https://imgur.com/gallery/tFxyX

If you're having too much trouble at this stage then maybe leave the colouring for later.
Worry about form and silhouette first, you can sketch just the outlines of the flames (using pencil & paper, or directly with black pixels).

Your flames look too symmetrical*, I'd try adding more chaos.

* When compared with real fire, reference.