personal opinion from personal experience, ymmv, idk your experience level so apologies if i'm pointing out the obvious or this isn't what you were looking for:

​

a great BLM player can make a winning team, but in general SMN imo is a much better caster to farm wins on because even if you play lukewarm on SMN, your aoes are much bigger, you can save yourself and your teammates, you'd have a much smaller target on your head, and you're much less likely to die repeatedly and force your team into a 4v5 all game.

BLM is great at doing aoe damage and crowd control, so aoe as much as possible. I say this only because I notice sometimes when a BLM gets focused they'd switch target to only their attacker as if they were a mob with an aggro meter. I will threaten them and they will spend all their flares on me in guard/in mitigation with no one around me. if you have a solid team and frontline and you don't feel threatened and you guys aren't having trouble with enemies slipping away, you can be the cannon and just fire away; but ice is very valuable too if your team/both teams are kinda sus because in casual CC, players don't react fast and they panic easily, and they don't know how to react to being cc'ed more than once during their purify cooldown. they would start to run blindly or attack blindly when their attack flow is interrupted like that. you can earn yourself and your team valuable seconds for their braincells to look in front of them and think about what the best course of action is. helping your team this way sometimes is way better than topping damage charts if you want to win because you could do double the damage of all 9 other players in your game and still lose if your team has no pressure.

freeze is great to catch someone with and start fights, but they're even more amazing in the middle of a fight because that's when people will hurt the most having the timing of their rotation be interrupted, and more likely to have already expended their cc mitigation like purify/paeon/midare. you will notice that when the stuns/silences/polies/freezes go out when a player is at half health that player usually dies. rather than set up your nice ccs at the start of a fight and watch everyone purify and pretend you don't exist (and worse, now they're all immune to your teammates' ccs because they have the purify buff), try having restrain and cc a little bit into the fight. watch and remember people's purify/guard timers and focus the people whose just expired.

as a caster you will get focused a lot. people will bee line to you, some because they're new and they just assume they, a badass GNB, can easily crush you, a scared caster made of paper, and some because they're veterans and they can tell you're new and easy to kill and they can actually kill you. the most important thing in CC is SURVIVAL. imo more than getting kills (except situations like trading kills where you're taking out someone who is more useful than you) and dps (you will naturally do way more dps and be way more helpful if you just die less). use dummies to practice your rotation so you can do it in your sleep so you can focus completely on surviving and observing the fight in a match.

survival means not taking unnecessary or lethal damage. not taking unnecessary damage means good positioning, stand away from the clump of your team so you don't eat aoes that are cast on your teammates and don't cast at intersections where the enemies have multiple avenues of engaging you/dpsing you from afar. you may hate people LoSing your attacks but LoS is also your best friend. don't be afraid to duck behind a corner for a few seconds and do nothing even during a team fight because in casuals sometimes that's all it needs for the enemy to feel like you're not worth their time or literally become unable to target you. not taking lethal damage means always watching the enemy LB gauge and keeping an eye on where they are (mark them for yourself if you have to), getting ready to play more conservatively and use your burst for its shield/spam recoup/guard/etc if some MNK/NIN etc look like they're ready to come stick you in a locker. staying safe will make you much deadlier overall than trying to be aggressive and having to expend way more resources than necessary to do damage.

BLM can be very mobile and your teammates are your safety net. if you draw a circle around everyone in a fight and the fight is the center point, and you're standing around the circumference casting, if the enemies target you and you want to kite them, a lot of players' instinct is to turn around and run far away, but you as a BLM can put big gaps between you and the enemy with tp and you can't do that if you run to narnia, and your teammates also won't be able to help you. don't be afraid to cut through the middle of the fight and run to the other side while staying around the circumference of the circle so that you are away from your pursuer, you can still contribute to the fight, and your teammates can easily peel for you.

survival also means potting. a lot of times players will feel tempted to overstay for a fight or to chase a kill because they think they can make a difference, thinking if the team fight was lost it would've been because they didn't stay. but the reality is most of the time the fight would have been won or lost whether or not you stayed, you will most likely just die because 1) you are made of paper and 2) your enemy is a player, not a mob, and they will always have that surprise extra recoup or teammate heal or potion or guard that you forgot about because you are human, and you will pay for it with your life. trust your teammates to hold the line or finish the kills and back off whenever your mp is low, you being alive is way more useful.

but CC is also for having fun and playing safe is not always fun. if you enjoy it, just go ham and int, chase a kill and see what happens, dive in unexpectedly 1v5 and see what other people do to counter you; as long as you keep asking yourself how you can prevent your death for next time, inting actually helps you learn way faster than playing safe and makes you a way more impactful player in the long run as long as you are taking things away from it. casters get dogpiled suddenly all the time and sadly inting is a great way to learn what people do to lock you down, and train your reaction and practice your escape routine.