To add an email address either to the Blacklist or Whitelist, you will need to login to your Office 365 account via the web application.
Once logged in you will be prompted with the following screen, select the Icon that says Outlook.
Selecting Outlook the next screen that appears will be your email inbox.
The next step is to click the cog wheel icon at the top right of your inbox screen which will expand a drop down list, select Options.
This will take you to the next screen listing many settings options, the one we are interested in can be found under Accounts - Block or Allow, click Block and Allow.
You will have have two options, Safe senders and recipients and Blocked senders.
To add users to the safe list, enter the email address under the Safe senders and recipients box and click the + icon once done. The same for blacklisting unwanted emails by added them to the Block senders list.