I have poured concrete basement with two rows of cinderblock at the
top. During the heavy wind driven rain the water starts to seep
through the mortar where cinderblocks connect with concrete. This
happens despite the fact that the level of connection is above the
ground by about 2 ft (exposed area of the basement wall above ground).

From the outside the level of connection is slightly visible as a
tiniest hairline crack. The crack stays wet on the outside after the
rains are gone for a couple of hours or so indicating it's more wet
than the rest of the wall.

How can I waterproof or seal the mortar (or that hairline crack) from
the outside so that the water does not seep through it?

What substance can I use? What color is that substance? (The house is
grey there.)

Thanks!