Is there only client staking possible or are there also masternodes? If so, how much coins are needed for a masternode? If not, what is the daily return for staking?
Blackcoin maybe needn't something like masternodes. It's running on Proof of Stake static reward.
what does that mean exactely? for example: i hold 1000 coins in the blackcoin wallet. how much reward will i get a day?
does it have to be online all the time or is it like claming gas in NEO ?
Blackcoin has fixed inflation of 1.5 new BLK with a block time of roughly 64 seconds. That's approx 739,631.25 new BLK per year. Nothing anyone does will change that.
You will only ever receive 1.5BLK for staking a block (+ tx fees from transactions in the block).
Your chance of staking is dependent on the amount of coins you hold and stake proportionate to how many the whole network is staking. It's like a lottery. The fewer coins are staking across the network, the more, %-wise you will receive per year on your coins if you stake 24/7. Yes you have to keep it on. BLK will work fine on a Raspberry Pi or similar device which costs approx $3 a year to run.
With 1000 coins, expect to stake about once every 10 days.