It is possible to modify the default execution of FormatBlock command by assigning handler to the ClientCommandExecuting event, e.g.:
function editorCommandExecuting(sender, args)
if (args.get_commandName() == "FormatBlock")
args.set_cancel(true);//cancel the default execution
//add code to imeplement custom execution
Please note that, the FormatBlock command is very complex. It involves selection modifications, examining the DOM etc. It will be very difficult to modify its output to match the requirements.
If you want to apply text decorations to the inserted heading you can achieve this using the approach demonstrated in the following live demo:
Editor / Paragraph Styles
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