The blue halo is often referred to as purple fringing. It is caused by chromatic aberration. According to Wikipedia:

There are two types of chromatic aberration: axial (longitudinal), and transverse (lateral). Axial aberration occurs when different wavelengths of light are focused at different distances from the lens, i.e., different points on the optical axis (focus shift). Longitudinal aberration is typical at long focal lengths. Transverse aberration occurs when different wavelengths are focused at different positions in the focal plane (because the magnification and/or distortion of the lens also varies with wavelength; indicated in graphs as (change in) focus length). Lateral aberration is typical at short focal lengths.

As @MichaelClark states (emphasis added):

You'll probably find that most of the axial CA causing this is provided by the cheap TC. Using the lens bare and cropping the snot out of it will probably give much less purple fringing. Even higher quality TCs tend to demonstrate purple fringing when there is strong backlight contrasted with dark subjects.

There are a variety of Q&As on this site about the topic, many of which explain the cause and how to avoid it:

• Chromatic Aberration

• Color Fringing