Well we can only go on the evidence, such as it is, and there's...

Well we can only go on the evidence, such as it is, and there's no evidence whatsoever that any coronavirus can be transmitted via blood. On the other hand, there is an increasing body of evidence that Cuvid19 can potentially be treated by a plasma transfusion from a patient who had contracted the disease, and subsequently recovered (i.e. whose plasma contained the antibodies).

As to why people would "take the risk of giving blood, or plasma, if there's a chance they might get infected', I would suggest a) that the likelihood of getting infected in a sterlile CSL managed donation clinic is no higher than the risk of getting infected anywhere else, and probably lower, and b) 'donors' get paid about US$50 for giving blood or plasma, and this sum could presumably be increased if it seemed necessary to boost the numbers.