The definition for blues scale says the fifth pitch is diminished, but you can see almost everywhere that some sources says it is diminished 5th and augmented 4th. Are these "additional" scales strictly defined? Is it exactly diminished 5th, or for simplifying notation is it whatever depending on melody lines? Is that the same case as Dflat is not Csharp?
I'm expanding on jjmusicnotes's answer in the comments, though with a slightly different answer.
Wherever possible, it's a good general rule to only have one of each pitch name in a scale. In the G Major scale, for instance, the last two pitches are not Gb and G, but rather F# and G; having two types of G in the first instance gets thorny really quickly, and since you have an F to use up, you may as well make that pitch F#.
That's basically what's happening in this C blues scale:
It wouldn't really be incorrect if you use Gb, but it's easy to see why F# would be so much easier sometimes:
Isn't that first measure much easier to deal with than the second one? They're both exactly the same, just spelled differently.
jjmusicnotes said to use F# when you're going up the scale, Gb when you're doing down. But sometimes that direction isn't really clear, so I'd recommend just sticking with F# (or, more generally, the raised fourth scale degree).