I primarily use a couple of apps I've downloaded from the Play store, "Dólar Blue Hoy" and "Ambito Dólar". Both apps have excellent ratings. The apps show the current blue and MEP rates (among others), and have built in calculators to do conversions while shopping and whatnot.
The only thing that's mildly frustrating is that the Visa credit card rate never exactly matches any of the various rates shown in the apps, so doing conversions while shopping is always off by a bit. For example, as I write this the Visa rate is 876, and the apps report that Blue average is 930, MEP is 907, and tarjeta/turista is 966 (not sure what tarjeta is supposed to be since it always differs from the Visa rate by a large amount). So if I want to make a large purchase and want to know exactly how much my card will be charged in USD, the in-app calculators will get me close with the MEP rate, but I still have to rely on Visa's website to pin it down to exact pennies.
I should email the app developers and see if they're able and willing to add Visa as one of their displayed exchange rates, would make things a bit easier.