Poo after banana!

I haven't as I'm waiting till 6 months. But this is normal it's a transition poo. Also, you shouldn't introduce fruits so early you should start with bitter veg as you don't want to wake the sweet taste buds thus early

I would echo @Fatima. It’s worth exploring all their veggies first as fruit will taste much nicer and may become fussier over time x

Banana is notorious for black streaks in poo. But as above id go for veggies first if you're weaning early. A lot of purees in shops also have apple or pear as first ingredient even if it's a veggie puree as it's cheaper but makes it sweet.