Think of it from your prospects point of view too. If I have to sign up just to read a blog post, I will not. So, putting a pop up here is not a good idea. Also, I might already be a customer just coming to learn more about your product. Do you really want leads that are not well qualified? Because that is what you will get as there will be visitors who might not be right customers for your product. Instead why not build some other content like whitepapers, video, presentation, etc. that a prospect might be willing to sign up for. That will give you better qualified leads.
I help B2B clients build their product marketing strategies. Happy to chat if you have more questions.