Hard to say without knowing more about your account.

15 blues isn't, by itself, gonna do much for you in the fusion.  On the other hand the chances of another lego during the 2x are rather slim.

id say if you think you can complete the fusion you absolutely should do that.  Otherwise save them till you can complete one.  That's unless you just need more leggos right now and want to take the chance.  But with duchess your game just changed and there will always be another 2x.

Ok, thanks. Can you  me how should I preper for that fusion ? To tell the truth it will be my first fusion event and I don't know what to expect.

You're best checking out streamers who give tips on these  - Hellhades for example did a video or two yesterday or day before I think on how to approach these traditional ones. They're harder than fragment ones because you have to get rares, level them and ascend them, then fuse those to epics and so the same. Lots of time and resources including potions needed.

Extremely strong fusion but to add on to what angwil, dram said: 

you'll want a healthy supply of shards going into it, ability to beat dungeon 20s.  you'll almost always want to be already getting top chest on nm/unm clan boss every day. the resource generation there is quite key for fusions. 

more details about where your account is at would be helpful, but the earlier you are the more apt I'd be to just open ancients now. they're not worth much in a summon rush event and if you're still lacking in epics then this is a perfectly great time to open. 

That is my rooster 

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Zero question - go for the fusion. This champ is going to be forever on your FK team.