All of the aura colors are directly linked to the different chakras, with colors corresponding to each. In this case, blue is related to the throat chakra, which governs communication, expression, and truth.
If the blue in your aura is bright and clear, this is a sign your throat chakra is balanced and flowing. If the blue looks slightly murky, neon, or washed out, it could mean you're struggling to express yourself and your truth. (For this, Kaiser has a mantra to amplify blue energy: My voice is my vehicle to create harmony and peace in the world, for myself and others.)
And if you have reason to believe your throat chakra is experiencing some difficulty, reiki or other energy healing could be beneficial. As Hira notes, "Since the blue aura is connected to the throat chakra, reiki can help someone to increase their self-expression and communication using their voice."
Additionally, blue is often associated with balance and serenity, so if you've just had reiki, Kaiser notes there's a good chance you may have more blue in your aura afterward.