How can I use single search box with autocomplete for all the users on site and all the content of particular content type like blogs.
No need to create two views this can be achieved by modifying the query if you are using views. You can easily alter the query, if you are using the custom form then you can use Form API with autocomplete and in autocomplete callback you can do whatever you want. I have solved this using the autocomplete and its callback using FAPI
If you are using Exposed filters you can use the Better Exposed Filters module, this allows you to combine serveral exposed filters into one filter.
How this works is described here: combine several exposed filters into one exposed (search) form?