Is a blood pressure of 90/50 ok?

Yes. The reading is on the lower side but if you feel ok i.e. Don't feel dizzy or weak and normally run lower blood pressures, the numbers are ok.
Very Low Nl, Maybe. Since the pulse pressure (90-50=40 mmhg) is reasonable, possibly accurate & possibly ok if heart output good, can function well & not dizzy on standing. Keep in mind: 90 read/estimated from only one heart/pulse beat, 50 only one heart/pulse beat & not the same heart/pulse beat + values can (& commonly do) change rapidly. Low bps correlate with lower rates cv disease progression & event rates.