Gender, age, location, what they do, what they like and do not like, what places they go, their lifestyle. Clearly figure out your target audience and understand what bloggers do have it. And then do the collaboration to exchange the audience or paid advertisement with that blogger.
If it is a paid advertisement, it is you who should decide, think trough and offer the format of the advertisement. It is you who should offer the options, so the pr would be much more effective!