A few months back, I wrote a post detailing an emerging sub-style of Mori Girl called Black Forest Mori. The style itself is a darker take on the popular Mori kei (mori meaning "forest" in Japanese, so Mori Girl aesthetics are based on a girl who lives in a forest). When I wrote the original post I was, and still am, besotted with the ideas behind Black Forest Mori but what I was most interested in was the idea of it being a "dark" version of an already existing style; a witchy, more gothic twist on a previously rather innocent fashion. I also mentioned that I'd originally stumbled across the term "Black Forest Mori" while I was searching for a style I could wear on a more daily basis rather than lolita. I adore the asthetics of OTT sweet lolita but it isn't something I would feel comfortable wearing, so I favour gothic, classic and old school sweet coordinates instead (of course, I adore guro-sweet hybrids but that is for another post entirely !). However, learning that there could actually be "darker" versions of existing styles had got things ticking in my head. Could there possibly be a darker version of sweet lolita ? Something to sweet lolita as Black Forest is to Mori Girl ? I didn't have to wait very long for my answer. A few weeks later, the term "bittersweet lolita" cropped up on my Tumblr dash and at once I was interested in finding out more.
A quick google search of the name lead me to this post on the egl community on livejournal, written by lilusako. As far as I know, this is where the term originates and the OP has given the Wiki definition of bittersweet to epitomise her ideas of the style:
"Bittersweet refers to a combination of the standard tastes of sweetness and bitterness, and is often used as a metaphor for experiences which have elements of both happiness and sadness."
A dark colour palette but with sweet-themed prints and accessories ? Perfect. I was eager to find out more - everything described sounded to me like my ideal lolita coord. It was almost as if this was a lolita sub-style I'd been trying to achieve without really realising it. It's a nice middle ground between the cutesy prints of sweet and the dark decedance of gothic. Hello Teacup described the style perfectly in her brilliant post - "It's darker then sweet but sweeter then gothic". I love the idea of replacing a pastel colour palette in sweet lolita with darker colours but while retaining innocent themes within prints such as teddy bears, butterflies, princesses and candy. For me, this is an absolutely perfect combination and a way for me to be able to wear something sweet lolita releated but without venturing from my comfort zone of darker shades.
Outlining the differences between Sweet & Bittersweet Lolita (click for source)
Another idea that is really appealing to me about the bittersweet style refers back to how I feel about Black Forest Mori. I almost feel that a bittersweet coord is something of an "evil twin" to the alternative sugary sweetness; a gloomier, perhaps spookier type of girl. One of my favourite things to see in lolita is twin coordinates and I would be so interested in doing a "good twin, bad twin" version with sweet and bittersweet ! I would love to see examples if anybody has done this already. I've thrown together a quick and very short bittersweet lolita checklist (mostly because I couldn't find one that anyone had already written !) I am by no means claiming this is the "correct" way to wear bittersweet and if anybody has any suggestions I would love to hear them !
~ Replaces light, pastel base colours in coordinates with darker colours such as black, dark red or navy.
~ Uses prints commonly seen in sweet lolita such as teddy bears, florals, hearts and animals.
~ Frills, bows and lace are still present.
~ Retains the all-important "cupcake" silhouette of sweet lolita.
~ Because there is still an element of sweetness to coords, accessories can also be sweet-themed.
~ Make up could be darker than sweet & darker coloured wigs could also be used (eg, dark pinks, black)
www.lolita-tips.tumblr.com
And finally, here are some pieces that could be used in a bittersweet coord (all images found on hellolace.net).
There are still a few little bits and pieces I need to go out and buy before I feel that everything is ~completely~ finished but when I've done that I'll post some photos. Everything is very pink and cute, my boyfriend decided that my bedroom is "a testament to my life as an eternal eight year old" haha. I've moved into the city now, which is so much better than where I was living before. It also means I can eat out all the time in amazing places like a delicious local Japanese place ~
Speaking of mori-esque things, thank you guys so much for the response to my black forest mori post ! I've been inspired by this to write another, much longer and more detailed post about two other dark versions of popular styles - a dark dolly kei inspired by the more Romanian/bohemian aspects of the style, and also bittersweet lolita which is absolutely one of my favourite things at the moment ! So I promise I will start working on that really soon 
I'll post a review once I'm done. Until then, I'm off to fight monsters !
