On the one hand, their settings are very different--Korean historical versus Chinese 20th century--but both tell epic tales over years of the characters' lives and interweave the maturing of the country with the maturing of the characters.

Both take a bit of time to grow attached to the characters but then you become SO sympathetic with them that the feels are intense.

Both have some romance but it is not the only ingredient (it is definitely featured more in My Dearest). Both are very well acted and nicely paced.

Both leave you feeling like you experienced something meaningful.

Both are elaborate xianxia dramas with excellent world building and the same epic fantasy feel. If you enjoyed being caught up in one and are feeling lost when it ends, the other will give you the same absorbing fantasy feeling. Also,
--The leads go through several different arcs in their relationship due to external forces but always find their way to one another again.
--Both are smart dramas, where characters make decisions consistent with their motivations and characters.
--Both have a well thought-out storyline that ends satisfactorily for our leads (and without being choppy or rushed like many cdramas), and ties up all the side stories we've invested in along the way.
--Eternal Love has more funny moments where Blue Whisper is more dreamy.
--The music is noticeably good and adds to the ambiance.
Note: The ML in Blue Whisper is a merman, but the whole story takes place on land, so he ends up acting just like the other powerful immortals.

Same wuxia genre and a nicely thought-out plot progression where the resolution of one mystery/arc opens up the next mystery/problem to be solved arc. Suspense keeps driving the plot forward, but neither drama is overly intense.
Both have playful but intelligent male leads.
Romance is sweet but understated.
Be warned, JOL only resolves partially as a Season 2 was planned; LOF has a nice satisfying ending.

Both modern C-dramas feature a mature, mutually supportive and heartwarming relationship between the leads. They face potential misunderstandings and relationship challenges by trusting one another, rather than with tropey fights and breakups. The genres are somewhat different (Love o2o has more light silliness interspersed with the romance while Forever is more sedate and focused) but both will have you smiling throughout.

Awesome wuxia
Nicely paced suspense plot progression -- Both follow a series of smaller mysteries/arcs that build upon one another until the overarching mystery is solved.
Physically weak but mentally strong ML protected by butt-kicking friends/FL.
Some bromance

Both have heart-pounding romantic moments and a sizzling chemistry between the leads. Both avoid straying too far into dumb plot devices to drive the couple apart. Both offer a pleasant ride without too much angst. If you like the feel of one, you'll definitely like the other.

So many similarities--bullying themes, bittersweet love triangle with serious SLS, aloof but lonely ML, bromance--but mostly they're similar because they both have the feel of a shoujo manga/webtoon with the requisite romantic twists and turns and tropes. BOF is a bit sillier at times but both deliver the feels on a big scale.

Not quite as epic as BOF, but if you enjoyed watching the normally arrogant and self-assured Joon Pyo become insecure and goofy as he tried to express his love, then you will like this one. The male lead has a similar vibe and you WILL laugh out loud watching him. Nice little love triangle and some good-looking guys round out the similarities.