Context
US$ 25 is a bill or a small monthly amount; it is appropriate to compare the bid rate The one who buys you the dollars pays the bid rate; The one who sells them to you charges the one who sells them.
For historical reference: a week ago (06/11) US$ 25 cost $ 38.875,00 (+0,96%); a month ago, $ 38.750,00 (+1,29%); over the last 30 days the range was $ 38.625,00 to $ 39.375,00 (daily selling data from argentinadatos.com).
Keep in mind: It is an illegal, unbacked market: the published price is a reference average and each trade may vary, plus there is a risk of counterfeit bills or scams.
The other way around? Passing pesos to blue dollar.