Oh dear, thankfully he's okay! Don't beat yourself over it, kids fall and bruise themselves all the time - so you don't have to worry too much :)
The blue black should go away in a few days, as long as he's still happy and active it's fine!
On a side note, you may want to consider getting another alternative since he's able to climb out of the baby cot himself now - it could be dangerous with or without supervision :)
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