Thank you, Daphne, for the additional information. As far as GP integrations, the tools are vast across the channel so each GP partner will probably have them only version of this answer.
Assuming your question isn't about a web service itself (as GP has one) but more along the lines of how you get the BlackLine data into the GP database, the internal tool I like is eConnect as it is based on SQL stored procedures and integrates into GP at the business-logic logic. However, it does not have a user interface. When a client needs a user interface, my favorite recommendation when the data is coming in an Excel-friendly manner (like CSV), is TitaniumGP.com. It gives the user the comfort of Excel and handles all the behind-the-scenes eConnect magic.
The last time I worked specifically with a Blackline integration, I think GP's Integration Manager was the tool used (as the customer already had it licensed). With the change in licensing for Integration Manager, it is rarely (perhaps, never) my go-to recommendation when it is not already licensed.
The Dynamics GP 2013 Architecture White Paper that I think Beat mentioned, has a few pages on Data Integration that describes the Microsoft toolset.
Other tools we see at client site come from Jitterbit and eOne. I'll let others chime in with their favorite tools ...
[P.S. - BlackLine must be in outreach/growth mode: I ran across their BlackLine AR product just yesterday!]