When applying a link to text it is standard HTML for it to change blue, this is not something we can prevent.
When trying to change it back to the colour you would like it to be, you can either highlight the link and select the colour from the foreground colour selection box (at the top of the editor) or you can change it in the HTML code as instructed below:
Switching to HTML view
Click on the HTML button at the bottom of the editor to switch from design view to viewing the HTML code.
Switch back to design view
Switch back to design view to check you are happy with the colour of your link.
Please Note: You can remove any colour by using the breadcrumb string.