The Guardians are the eldest race of sentient beings, right?  So how can they not know what's going on?  Apparently, if a Green Lantern works with another colored Lantern then the combination can defeat a Black Lantern.  So why did Ganthet want to send the Blue Lantern Corps to find the Indigo Tribe?  It's something that never happened but still, wouldn't it make more sense to keep the blues with the greens if that's what it took to defeat the Black Lantern Corps. Why have them go their separate ways?  Or does the combination of blue and indigo do something that green cannot? I find it hard to believe that the longest living immortals in the universe would fumble the ball like this.  Do they really not know or are they even keeping important info like this to themselves?

Interesting question. The answer would be that Ganthet and the other guardians don't really get along nicely these days. Blues hanging around the greens might just produce unnecessary friction. Besides, it takes all the lights to unite to defeat the black for good. So it really does no good if the blues and greens are hanging out if one light is still missing. 
 

As for the guardians themselves, it is a long list of reasons presented in since SCW. Long story short, rather than working to overcome the blackest night, the Guardians sought to prevent it all together. To do this, they hid all the other lights in order to prevent the light war which would trigger the blackest night. 
 
They kept the green light for themselves because green is the only light which does not affect its user. All the others were deemed too dangerous and thus were hidden, every trace of them erased. They have an innate distrust to all the other light, and since this is something the Guardians have been working on for three billion years, after such time I'd imagine it's pretty hard to just admit you were wrong.
 
Hope that answers your question.

Well, yes and no, the combined might of all the corps working together may be able to defeat the Black Lantern Corps and perhaps even it's source for good, but as seen when Indigo-1 'synched' with Hal Jordan, the result was the disconnection of the Black Lantern ring from it's host body.  So, y'know, that's something that could come in handy right there. 
 
And just cuz I need to say it, I find it highly odd that the new corps are all being created right now.  I can understand if they were being discovered in the present time, but they were all created like months apart from each other (with the exception of Orange and, perhaps, Indigo)  Red, Yellow, Blue, and Violet are all newly made...in DC time, not real time.   I just had to get that out. ;)

" Well, yes and no, the combined might of all the corps working together may be able to defeat the Black Lantern Corps and perhaps even it's source for good, but as seen when Indigo-1 'synched' with Hal Jordan, the result was the disconnection of the Black Lantern ring from it's host body.  So, y'know, that's something that could come in handy right there."

"And just cuz I need to say it, I find it highly odd that the new corps are all being created right now.  I can understand if they were being discovered in the present time, but they were all created like months apart from each other (with the exception of Orange and, perhaps, Indigo)  Red, Yellow, Blue, and Violet are all newly made.  I just had to get that out. ;) "

Yeah, I was speaking of the canon appearances of the Corps.  The Orange, Green, and Indigo, however well-known or unknown they are to the universe at large, are the only ones that have been around awhile.  It took Atrocitus until recently to find a way to create the Red Lantern Battery, the Zamarons finally got it together and forged the star sapphires into rings, Sinestro and Qwardian rings have been around for awhile now, and Ganthet and Sayd did their thing after leaving the Guardians.  Those four are the newbie Corps...canon-wise, one or two of those could have been made to be a little older, hehe.