Even if they are still filming Valentine’s Day, it doesn’t mean Blaine is in it.
I’m with everyone else in hoping for at least an appearance in Heart. I’m keeping my fingers crossed, but I’m going for hopeful realism. Since well before break, they’ve been shooting two episodes at a time because they are running behind. It’s very possible they could be starting in on 3x14 in addition to 3x13 (Heart). (I don’t think they’re still filming 3x12 aka The Spanish Teacher.)
It’s very possible they were fully expecting to be done filming Heart before Darren returned and didn’t include him in the script. It would now make it easier to have a second unit on the following episode with Blaine scenes while they’re finishing off Heart with other characters.
Now, it is also possible that Blaine wasn’t included in the script, and they added a scene in when they realized they could. It’s happened before. But given the sources saying he’s not in the episode, it’s pretty likely he actually won’t be.
TL;DR: I assume they are still filming 3x13 aka Heart aka the Valentine’s Day ep. While Darren will be back, it doesn’t mean he’s in the ep. I do still hope he is, but I’m preparing myself to not be disappointed.