I'd always assumed that the name 'blazer' had originated with the fact that they were typically
emblazoned with the crests and colours of the schools, clubs and societies they represented.
Buttons seem wholly incidental.
The '-er' suffix may denote the object performing the verb ('player', 'dancer' or 'manager'), but here it's more likely to be an 'Oxford '-er' (such as 'rugger', 'soccer', 'Divvers' 'brekker', champers--or my favourite: 'Honkers and Shankers').
https://en.wikipedia.org/wiki/Oxford_"-er"