How bad are the blackflies around the NH seacoast area--Portsmouth, Hampton, Stratham, etc.--and about 10 or so miles inland? Are they as bad as they can be farther inland? What are the worst months?

Thanks!

The mouches noires are not sea coast dwellers they need clean running water.The warm days of spring bring out the little buggers,lasting to after mid June.
On the coast you will contend with greenheads and salt marsh mosquitoes.
These flesh eaters last all summer.
Stay indoors or off shore to avoid them.

Well I grew up in Portsmouth and really never noticed any black flies.

The greenheads are mostly localized in the Plum Island area of Newburyport MA so they're not a big problem in NH. A few wanderers but nothing unmanageable.

So to answer your question, I wouldn't worry.

Unless you are planning to hike or fish near rivers, streams, waterfalls, you are not likely to encounter any black flies. Even then, you may not.

Greenheads on Plum Island usually are in July and disappear in August.

If you plan on driving around, shopping, and eating indoors, you should have any problem no matter what time of year. Downtown Portsmouth would not be a problem. I tend to only notice them when I'm trying to work in the yard. Generally late May until early June I stay out of the woods.