When purchasing graphics, yes you can use it on your website in a blog post. The license specifies that it can be used in a single end product, and in your case, the end product is your website, so you can use the graphic as many times as you want within your own website. However, let’s say you wanted to create another website and use the same graphic, in that case, you’d need to purchase another license.
As for using graphics provided in a powerpoint template, I’m not sure. I would assume it would fall under the same usage as what I mentioned above, but you might want to contact Envato Support to get a clear definitive answer.