Then why does it have a large number of democrat candidates as recipients of ActBlue money and no other political party candidates of note?
Why does BLM use ActBlue to process the donation and not PayPal or American Express?
ActBlue is listed as a left-leaning PAC to help Democrat candidates, so why is BLM using them as a processor and not a neutral one?
Because they're explicitly a leftist organization. BLM presumably uses them because they're an effective fund raising tool, as well as being fellow travelers.
Found a better reference than Wikipedia:
https://www.influencewatch.org/political-party/actblue-pac/https://www.influencewatch.org/non-profit/actblue-civics/https://www.influencewatch.org/non-profit/actblue-charities/https://www.influencewatch.org/for-profit/actblue-llc/ActBlue started out as a website in 2004, created specifically to help Democratic candidates collect funds from a lot of smaller donors. They provide a number services, like allowing donors to save their credit card and use it to donate to multiple candidates, automating the filling out of fund raising forms, a variety of tools for optimizing fund raising campaigns, as well as relatively low transaction fees. The Republicans don't have an equivalent organization, which hobbles their ability to raise money from lots of small donors.
ActBlue is broken into a three separate branches, each supporting a different type of legal entity. One of the three is a PAC, the other two are 501(c)3 and 501(c)4 charitable/social welfare organizations, and each supports their own kind. All three provide the same basic fund raising platform services, and the funds they collect (minus transaction fees) are passed through to the entities chosen by the donors, rather than being distributed at ActBlue's discretion. All the groups they support are progressive, and the ones I recognize include the charities GLAAD, Mother Jones, and the Southern Poverty Law Center; the social welfare groups ACLU, NOW, and Planned Parenthood; and a bunch of PACs (which I don't know by name).
There's also a for-profit branch (an LLC), but it's unclear what its role is. Though it may be the original -- the three branches above were founded between 2009 and 2015, but the website dates to 2004.
This link has some more detail about the services ActBlue provides and where the money goes:
https://www.realclearpolitics.com/articles/2018/01/23/actblue_fundraising_platform_strikes_gold_--_for_liberals_136068.html"ActBlue works with candidates at the local, state, and national levels -- from school board races to presidential campaigns -- to squeeze every dollar out of their email fundraising pleas or the ubiquitous 'Donate' button on their websites," Kroll wrote. "Engineers streamline the process of giving to a campaign or cause. They toy with typefaces, reduce load times, and adapt the product to all devices and operating systems. Like an Olympic sprinter in training, ActBlue obsesses over shaving off every millisecond."
[ActBlue] charges nothing for its services. (It takes 4 percent of every donation to cover credit card processing fees.) Operating costs are paid with tips left by donors and the occasional fundraising campaign.
The average contribution size [in 2017, including all 3 branches] was a very-"grassrootsy" $31.95. More than half of all donors gave for the first time in 2017, and just over 40 percent of all contributions were made from a mobile device.