Solutions Opaque for blacks, and I think there's a version for brighter colors that isn't as expensive. I believe both Imprintables and Stahls offers it.
Roland's redone their HTM design, but they sold it as a subpar product many years ago and it actually drove me away from heat transfer materials for quite a few months as it was the first one I tried/tested. Once bitten twice shy in that scenario.
You may also want to get some of the other transfer materials as the printable stuff isn't the easiest to work with, and large images ain't that fun with it either (such as full size photos). Even with our DTG and screen printing additions we still frequently will run names/numbers or simple lettering on darker colors through our cutter, some of the special effect films are cool and unique to vinyl cutters as well (I like the Fashion Film Electric from Stahls).