I dunno, never seen that before, maybe try using a health kit?
I have this problem too. I tried using a Med Kit and it doesn't do anything. To be honest, I kind've like it but, I can't tell how much HP I'm missing so I know if it's good to use a Med Kit or not.
I just got this glitch too. My second playthrough (new game plus - easy). Save point: [Escaping The City] battling the hunters with those two young brothers. I manually saved, rebooted my PlayStation and all is well again!
Using a health kit didn't help, the entire bar just shows as being black!