Suppose you want writing to hold on either slides or a specific website and not wanting to use the toolbar to erase all the text to navigate to the next thing. In that case, I suggest adding the extension Annotate: Web Annotations with Screen Sharing.  

Once installed, you will need to sign in and apply for a free educator account, allowing free access to utilize the black marker but be mindful that you have limited storage space with items you save for later use.