I heard this expression, and I think it means to talk seriously but I am not sure. Am I right?

"hablar en plata" means speak outright, without beating around the bush, telling it like it is or telling it short.
"en plata blanca" I suppose it's the same, but I'm not sure.

RAE
en plata.

1. loc. adv. coloq. Brevemente, sin rodeos ni circunloquios.
2. loc. adv. coloq. En sustancia, en resolución, en resumen.

Here explains the origin (in spanish)
http://www.1de3.com/portal/modules.php?name=News&file=article&sid=377
They say "hablar en plata"/"speak in silver" means to speak pure, because has an alquemic origin. All the alquemist tried to have pure gold or pure silver.
Saludos :D

Some context: (which makes me think "hablar en plata blanca" means "to speak frankly")

... No creo en estas cifras, hablando en "plata blanca" estas cifras deberia ser del doble y eso es que la cuarta parte de las personas ... [source (http://www.dinero.com/noticias-empleo/salto-desempleo/51794.aspx)]

Hablando en plata blanca como lo hacen en el Trinidad, creo que salí ganando con esta relación, a pesar que nunca trabajé en la zona, no tuve ninguna novia ... [source (http://www.critica.com.pa/archivo/021499/reporta.html)]

... porque esa es una condición para poderles soltar más dinero, ya que hablando en plata blanca, los más de 13000 millones que les acaban de otorgar, ... [source (http://www.latino-news.com/index.php?option=com_content&task=view&id=1097&Itemid=43)]

... Quizá sea una comparacíon desproporcionada pero hablando en plata blanca el amplio triunfo que obtuvo “Samuel el chico” ante “Enrique el ... [source (http://www.dhcolombia.info/spip.php?article440)]

Entonces, hablando en plata blanca, ¿a qué voy? Voy a que el cura Uribe es un tartufo que invoca el nombre de Dios en público y se refocila con viejas ... [source (http://www.galeon.com/ojosabiertos/aficiones1609812.html)]

Thanks Sosia and David. All the examples you gave indicate that hablando en plata (o plata blanca) is best translated as to speak frankly.

I have questions above every expression mentioned before in this forums, because I never heart those expression in my live, I'm sorry for ask you, but Where did you have got that expression? it's very common used on your country or your only have gotten the expression from internet, because last days, I wanted use an expression mentioned inside of the forums and the person never understood me, just I have a few joker with a partner of my office above that expression.

I have questions above every expression mentioned before in this forums, because I never heart those expression in my live, I'm sorry for ask you, but Where did you have got that expression? it's very common used on your country or your only have gotten the expression from internet, because last days, I wanted use an expression mentioned inside of the forums and the person never understood me, just I have a few joker with a partner of my office above that expression.
What expression did you use?

I have questions aboout the above expression mentioned in this forum, because I never heard that expression in my life. I'm sorry for ask you, but where did find that expression? Is it a very commonly used on your country or have you gotten the expression from internet? In the last few days, I wanted use an expression mentioned inside of the forums and nobody understood me. I have joked with a partner of my office about that expression.

Crotalito, encontré "hablar en plata blanco" en una carátula de un periódico escrito aquí en Nueva York. Sabes que a veces los dichos no se comunican in todos los paises. Sosia en España conoce el idioma pero sin la palabra "blanco" Unos ejemplos que dio David indica que se usa el dicho en Colombia, y El Diario (el periódico donde encontré el dicho) es pan-americano pero los fundadores y creo que la mayoria de los escritores son portorriqueños.

Ok, than you for your answer.