health isn't exactly a niche* though. Try by defining your market segment and your target audience better.
- niche is a very small market segment. The full market would be for example the general public or businesses. You then break it down into smaller, homogeneous market segments, for example young families in the UK (this could be further defined, and if you get to a very specifically defined market, then you've got a niche, for example young families, both working, child between 3 and 12, concerned about children's respiratory system) or small- to medium sized business that sell health-related products in the UK.
Once defined, find out more about them, what sort of information are they looking for, where, what's important to them and why, ... once you sorted that out you might be finding a couple of blogs that are not obviously related to "health" but are still worthwhile to build a relationship with - a mind map on your topic also helps, as would Google's keyword search tool.