Is blood (a sacrificial animal) obligatory for one who assumed the ihram for Hajj as a Mutamatti' (one performing Umrah then Hajj in the same journey) but was then unable to complete Hajj due to an illness that befell him after Umrah?

If the reality is that you have exited your Umrah and decided against Hajj, returning to your country before entering into Ihrām for it, then there is nothing upon you. This is because the Umrah has concluded, and you had not yet entered into Ihrām for Hajj.