Hi, Sateryn. Hmm, I've never had a problem with BB's liquid oxides bleeding. D&C colors will often bleed, but oxides should not. (BB's liquid black and liquid pink are oxides dispersed in glycerin.)
I wonder if it is possible that your layers are melting into each other and it looks like the color is bleeding? 140 degrees is awfully hot to pour - I usually pour around 115 degrees or else I melt everything and get wispy lines from the previous layer floating into my new layer.
As far as bases go, I wouldn't think it would matter which base you use, but it has been a long time since I've used anything other than SFIC base (which BB sells as their Bramble Berry base). I used LotP bases once when I bought a kit at my craft store just to see if I liked this whole soaping thing. After that, I stuck with SFIC bases. (SFIC is the manufacturer of the base.) I do think that BB's bases are of a much higher quality than LotP bases, so I think you will pleased with your purchase.
For using powdered oxides in melt-and-pour, I usually mix them with a bit of rubbing alcohol in a small bowl or cup. I use just enough alcohol to mix easily, usually five or six squirts with a spray bottle. If it's still too clumpy, just add another squirt or two until it gets just wet enough to mix.
Hope that helps! Have fun soaping!