Got to agree with JB here.
I've seen some large datasets collected on a selection of the major software's Blackjack games and those results I've seen have fallen within expectation.
I think the OP is also falling into the trap of using their own limited result set to judge fairness. It's great to hear that they've had some nice results when playing slots. Always happy to hear about a player winning. However, the simple truth about gambling is that the player is expected to lose. This is a basic mathematical fact. The longer you play the more likely you are to have lost money. Trying to determine which games are fair based on the mathematically small sample sizes any individual player experiences isn't possible as the sample sizes are almost always too small to draw a valid conclusion. The human brain is programmed to look for patterns and doesn't cope well with the realities of the volume of play it takes to actually conclude that even a low variance game like Blackjack is fair/unfair so people are inclined to jump to conclusions that aren't based on solid information. Slots games in general have a far higher House Edge than Blackjack. So if you've won while playing slots that's, in general, less likely than winning while playing Blackjack, though in both cases this is less likely than losing.
Alongside this, as JB pointed out, the rate of play online vastly exceed the rate of play at an offline of live game. More play = greater likelihood of loss. That said, online slots are just as fast if not faster.
And then there's bet sizing to consider. A lot of player will be higher on Blackjack than they will on slots. There's more of a tendency to select a bet size and remain relatively consistent when playing a slots game while there's more of a tendency to change up the bet sizes at Blackjack, moving up and down and sometime risking significantly more than you would on a single spin on a slots game. This creates higher variance.
My recommendations would be the following:
1) Try to slow your rate of play. This can be done in a variety of ways: take more breaks, take more time to consider what play you want to make, switch tables regularly, switch to live dealer games instead of RNG.
2) Choose games with more favourable rules. A bit of self advertisement here (apologies to all) but you can get a good idea of how favourable a game's rule set is here -
. Generally you want to avoid any game that doesn't have a 3:2 payout on Blackjack. That's a good starting point.
3) Learn Basic Strategy for the game you're playing. This will slow your rate of play down while you're learning and ensure you're playing against the lowest House Edge possible. It's important to realise (many players don't) that there isn't just one 'Basic Strategy'. BS is dependent on the rules you're playing against and often the number of decks in play, so there will be some different plays when you switch between games. Again the rules for any specific game can be found at the above link.
The bottom line is that the best way to lose less playing any online game is to bet less.
TP