Black Fever involves breakdown of RBC, so hemoglobin no longer remains tied to RBC and filtrated out of kidney as urine (that ultimately leads to death). So binding hemoglobin with some much larger macromolecule so that it cannot be filtered out of bllodstream is of utmost importance. As for viscosity-electrolytic balance-osmotic pressure maintenanace, the effect of ionic, covalent and macromolecular bonds on surface tension (extremely crucial for capillary, especially pulmonary) as well as viscosity of blood is required to be measured. The inert polymeric part of bllod substituent that doesn't bring about any harmful side reaction in liver (i.e. the chemical lab of body!) yet do not phase-separate from the suspension would be difficult to find out. And the blood substituent's fluid part needs to "respect" the blood-brain barrier as well. So, there are a lot of limitations to make up such a blood substituent!