Since blackflies actually prefer to feast on grazing animals such as cows or horses, you have no risk of being bitten by the insect at home. Rather outdoors near bodies of water you need to be careful.
Wear long clothes because the animals cannot bite through them, while normal mosquitoes simply stick their proboscis through them. Also a normal one mosquito repellent helps against black flies.
If you are bitten anyway, cooling will help, as with almost all insect injuries. Also mosquito plaster hydrocortisone-based anti-allergic creams or gels can also help, but the most important thing is not to scratch.