Personally, I think it's just the blood splatter itself that "jogged his memory" so to speak in a similar way as to how the blood splattered from the transmuted body Al's soul failed to latch onto.
Usually selective amnesia like Al's memory of everything involving the trauma itself can be brought back into one's conscious memories using a trigger which is usually something similar to something happening in the lost memory in the first place (I.e. saying an otherwise insignificant word in the right context that would explicitly be tied to that 1 memory or catching a smell similar to the environment that the event took place)
In this case, the trigger that would affect Al and tie his mind only to that incident is the death that HE experienced when his soul had been so violently and near immediately rejected from the body. (Which I'm pretty sure would be the trauma which caused his mind to suppress the whole memory in the first place. Who wouldn't be traumatized by actually dying?)
Also, the reason it happened when it splashed on his seal and not say...any other time they might have been fighting and gotten blood on his armor is because he can't feel it. The blood seal is a direct link to his soul, is the only thing grounding him to earth. Since his rejection occurred in a different body he's no longer attached to, it would stand to reason that such a trigger could only be felt directly by his soul or the body he harbors. The blood seal acts as that direct touch to his soul. And this is because blood is ones 'life fluid' and often thought to hold the soul and its direct energy within it (like our unique DNA and Mei's explanation of the dragon's pulse flowing in our bodies). But Al's seal is made with Edward's blood, not his own.
As such the defining reason it even managed to latch onto Al's specific soul and hold it so well and for so long is because their souls were mixed during the transmutation as Ed explains later. In brotherhood this is even easier to see with the brief flashback to Al inside the body they made when it bleeds out rather similarly under an outstretched hand.
That's my take on it anyways.