the blue background you are talking about is a feature within Adobe Reader or Adobe Acrobat Pro which you can highlight fillable fields. You can disable that functionality with the button at the top right of the PDF within your reader application.
Otherwise if you want to remove that blue background no matter what you will need to set the field's access to read only which cannot be editable.
If you do not want to set the field's access to read only but rather just remove the blue background, you will still need to set the field to read only BUT, to be able to re-edit the field, in the enter event of the field you will need to reset the field's access back to fillable
So the events with scripts for the field in question should be something like this