Are you reading this blog at that time?

The use of demonstrative pronouns can be tricky. If you want to refer to something that is current or happening now, use "this", not "that". I often hear Germans say (in meetings for instance) things like "We don't need to do anything at that time". Since they're refering to right now, they should say "We don't need to do anything at this time."

"At that time" is used to refer to a period of time other than the present. For example "I didn't like high school, but at that time I had no choice but to go." or "We'll go to France tomorrow and at that time will assess our options."