Well I suppose we could go through the whole gamut of small BOP as T2 has done.
However your description could fit several species. Size is deceptive. In fact, most people over estimate size of BOPs at a distance but tend to underestimate the size at close range when they are hunting. Male Merlin and Hobbies can look very dark grey almost black in flight as do Peregrine. Both Merlin and Hobby both hunt small birds as do Peregrine and Sparrowhawk (although Merlin really are very small (thrush sized) with exceptionally pointed wings. Hobby
frequently will hunt for dragonflies low over reed beds (unlike Sparrowhawk) and that, on habitat alone, would be my first guess - they will also chase birds if the opportunity arises during the course of hunting dragonflies over reeds and tend to swoop 'out of nowhere'. The flight then is more leisurely with wing tips appearing blunter and it's not unusual to then see them give a prolonged chase up into the air. (The hunting behaviour is typical of most of the species mentioned btw - most of them swoop down and pick off small birds!)