The black redstart is primarily a species of mainland Europe where it is common around towns and villages; curiously it seems quite at home in industrial situations and can be found around large factories and warehouses. In Britain there are apparently a few nesting pairs in south east England but they certainly do not breed in Dorset, well not yet anyway, they are a winter visitor. Whether our wintering birds are from central Europe or from elsewhere in Britain is not clear but the probability is that they are from more northerly locations. The numbers wintering in Dorset vary from year to year but the winter of 2018/9 seems to have been a good one with birds being reported from a number of places and not just solitary birds, sometimes two or three in one place which I think is a little unusual. In January 2019 there were seventy reports alone; that is not seventy birds but reported sightings.
The weekly reports show that black redstart begin to arrive in week 42 in mid-October and depending on weather conditions further afield the numbers seem to build during the winter and then inevitably decline in early spring and the are all gone by week 16 at the end of April.
Whilst some of our visiting birds frequent urban areas, there was one near Poole Quay for quite some time in January 2019 as well as reports from central Bournemouth, there is a tendency for them to inhabit rocky coastal habitat and so Portland is a popular spot as well as the cliffs along the Purbeck Coast with the caves at Tilly Whim, Durlston often hosting one. There are few reports from inland locations, they certainly prefer to be by the sea.
For list builders who want to see black redstart in Dorset the best bet is to monitor the news channels and head for where one is being seen regularly. They tend to stay in a favourable spot for a good while rather than move quickly on.