Brown spot on leaf

[Rightwing]

First grow. Need your feedback
I have 2 plants going. 21 days old auto blueberry. One of the plants had 2 leaves with a brown spot see below
Started in a jiffy peat pot transferred when it was 4 inches tall into happy frog (top)
with of (bottom) no muted other than topped with worm casting and bone meal
Only watering when meter shows dry
Thank you

 

[Johnny Mack]

What type of leaf? New growth or old and is it isolated to that one only.