If you use Microsoft Outlook in your computer, you should have this application configured to reply emails by using blue fonts (not black):
To change this, just go 'File' and select 'Options' on the left:
Then, go to 'Mail' and click on 'Stationery and Fonts...':
In the new window, you will have fonts configuration, select the one in the section 'Replying or forwarding messages' and set the fonts as you like:
Now, when you reply or forward an email you will use the font as you set it.