You can follow these steps to add Facebook Chat to Blogspot.
#1. First, access this free tool (free version is enough) and click Create widget for free
#2. Choose your desired chat layout and click Continue with this template .
#3. Next, enter your Facebook Username or Link then click Add to website for free
You can also set layout, style here
#4. Next, Click Publish on top right
#5. Choose Free Plan and click Copy Code .
#6. In the home page > Select Theme section
#7. At the current Theme, Click on Customize arrow
#8 . Edit HTML
#9. Scroll down to the bottom, paste the code right above tag
#10. If you receive this notification
Change the line data-elfsight-app-lazy to data-elfsight-app-lazy=“true”
#11. Result