Was able to do this by finding the fragment which controlled the colour of the show blocks border in ckeditor.js
Took a long time to find because ckeditor.js is a big file and minified but worth the effort as the show blocks function is valuable for the client when editing content.
Then had to change the text colour from grey to black in the small png images which display the html tags in the top corner of the show blocks area.
The client now sees the blue background in ckeditor which matches the background of the cms areas of the site and can also see the show blocks areas and their tags.
