An earlier writer gave 0.1 mg/m^3 as the US federal short term exposure limit.
However, a reference I have also gives a time weighted average of 0.05 mg/m^3.
You asked about blood levels. None of my references give blood levels. Since
the exposure you are describing is a respiratory exposure, engineering controls
and protective measures should keep the levels low enough to prevent detectable
blood levels. This is especially true of cobalt compounds, since cobalt is a
carcinogen.
Also be aware that cobalt has been reported to cause dermatitis in sensitive
individuals.
