Time is said to have an arrow, identifiable by the advance of entropy. In other words, if you look at two picture of the universe, with one less organized than the other, the less organized picture is the later picture because, as Stanislaw Lem so precisely put it, things fall apart. On the whole blues also has an arrow, and it is also easily identifiable: from a black American blues musician to an white British rock musician is the logical temporal order, and not the opposite. In other words, all other things being equal, the logical guess is that Eric Clapton took the solo from Albert King.
If this is so obvious, why belabor the point? Well informed blues addicts tend to be, shall we say, well informed. They know when a particular album was recorded, who the recording engineer was, what songs were edited, who the side musicians were, and much more. If, as happened around a different guitar solo, one expert claims that the Eric Clapton recording was before the Albert King recording, then we have to acknowledge that perhaps the blues arrow isn't as consistent as time's arrow. Of course another possibility is that, in this particular case, Albert King often performed his solo, which was heard and recorded by Eric Clapton before King recorded it himself. Anything is possible.
And if it's possible, it's most definitely a suitable subject for discussion.