To add a custom text to the top of the Blog Page, you can follow these.
#1. First, edit Blog Page > You will see an “Add a section” option

#2. Click Add a blank section

#3. Click Add Block

#4. Choose Text

#5. Then you can enter your desired text

#6. To reduce Text section height, you can click Edit Section

and Disable Fill screen option + Reduce number in Row Count
