I'm not a keyboard player so I'm not sure what the smaller notes in this score mean. Are they optional like an ossia or something to be played on a repeat or something else?
These are notes not present in the autograph, but suggested by the editor. The original score probably consisted only of the bass and the melody (with the melody having some chords at the end of phrases, which is quite a common thing for the period). So I suppose the editor suggested some additional notes to make the harmony less sparse.
I do not know anything about the piece, whether it was conceived as a keyboard piece or as a piece for a melody instrument with a continuo. As such it is hard to really estimate if these notes "should" be played. In the latter case it would not have been uncommon to have instruments playing improvised harmonic parts. In the first case probably not so much.
So we need to be clear: The small notes are not the original composer’s notation, but the editor’s notation. And to consider whether to play them is what the composer wanted would require more information on the piece and also historic performance practice, which I am not an expert in.
So as it goes: Play the notes if you do not care about that and you like them. If you do care about such stuff you’ll need to start reading up on historical performance practice of that particular time and area as well as the actual instrumentation of the piece.
The small notes are the editor's realization of a figured bass. At the time John Blow was composing, it was common that keyboard parts would be written as a bass line with figures indicating how the harmonies progressed. Below is an example from Blow's With Cheerful Hearts, mm. 11–12.
(Image source: IMSLP)
4Maybe, but is it an arrangement of a melody and bc or is a keyboard solo work? In the latter case I would find it unprobable to be a realization of bc.– LazyFeb 27 at 8:04