I think that black "ring" is just a baffle to help diffuse the light.
I just looked at my collection of CFLs that I have in the cabinet over the washing machine and most of the cheaper Chinese ones (TopLite and Sunpack) I got for free from Asian Supermarkets clearly say "Do not use in downlights or enclosed fixtures" on the box.
There are two that don't mention anything about use in recessed lights which are made by MaxLite and Greenlite but the Greenlite one says "Outdoor use requires an enclosed fixture" (most likely to keep the rain off)
I have one made by Philips; an old EarthLight Dimmable that says "This product is appropriate for use in recessed cans but use in recessed cans or totally enclosed fixtures may result in shortened life"
Above that, it says the same thing Greenlite says about using CFLs outside "Outdoor use requires an enclosed fixture"
I've read here that certain "Commercial Electric" models sold at Home Depot worked very well in enclosed fixtures. However, I think Home Depot stopped selling that brand; they replaced all the Commercial Electric CFLs at my Home Depot with a different brand; I think it's NuVision or something like that.
Commercial Electric brand CFLs were made by the Japanese based firm TCP (Technical Consumer Products) and there are several online retailers that sell TCP bulbs so one could look at those as an alternative.
I've heard some Panasonic Gen IV Compact Fluorescents (the ones with bare tubes; not the ones with the diffuser over the tubes) can be used in enclosed fixtures as well.
I remember during the Power Crises, the same Home Depot sold many different brands of CFLs; Lights of America, Philips, MaxLite, Panasonic, Sunpack, TCP, and maybe two others I don't remember but now they seem to have their own brand.