Model 1: Ebons are an alien race
Model 1.1: But they use the same four-letter DNA code as found on earth
Model 1.1.1: And they have a similar gametic system

Essentially this category indicates a species which is phenotypically similar to H. sapiens but achieves this through radically different genetic code. This is one "alien" scenario where human/ebon crosses may exist, although the chances are it will be inviable at the least: more likely would be pre-foetal abortion. To take a real-world example: It is possible to form cellular hybrids between human and mouse (or other animal) somatic cells. Somatic cells merely mean that they are are not sperm or eggs. However, when these are cultured, the result is a mass of undifferentiated cells, not a Geigeresque half-man half-mouse. In fact in this case the hybrids only exist because there is sufficient similarity between mouse and human genotypes. Say instead that you formed a hybrid between a human cell and a bacterial cell. Even if you somehow encouraged it to grow, multiply and differentaite it would almost certainly be such a confused mish-mash of code that the result would not live for very long.

To take an analogy from outside the field of genetics: Imagine you had played "Tie Fighter" and "Frontier", two computer games superficially similar in involving flying through space and shooting things, and you decided that you wanted a game which combined elements of both. If you took the source code for the original games in text format, and randomly combined the two, you would not get executable code. You'd get a Fatal Error.

The same would apply to ebons and humans. If the genetic systems were sufficiently compatible to allow cross-fertilisation, the resulting code - note the same word as the computer example - would not work.

This is not to say that it would not be possible for scientists in the World of Progress to intelligently combine alien and human DNA, but this inplies third-party manipulation and does not lie within the scope of this text.

Model 1.1.2: But they have different gametic systems
Bearing in mind that a species's precise gametic system is unique, it would be highly unlikely that an alien race would be able to pair successfully with a human. Given that an ebon's genitalia have roughly the same morphology as a human's, we could still assume that the roles will be the same, but perhaps the sperm-equivalent would be programmed to swim in a different way, or respond to different chemical signals. The result would mean that humans and ebons could copulate without any risk of pregnancy, which could be viewed as both good and bad.

Model 1.2: And use a different genetic code
Model 1.2.1: Which still relies on DNA, 4 letters making 3-letter codons
The human genetic language is made up of words called "codons", each made up three letters. The letters available are C, G, A and T; these stand for the chemicals which actually exist in the coding part of the DNA.

Each of the 64 possible codons has a specific meaning. However, these meanings are essentailly arbitrary and, given a point of evolution other than earth, could be different. The result would be that ebon DNA, being translated by human apparatus, would appear to have some meaning, but not spell out any functional phrases.

One further possibility here would be if the mechanisms for decoding DNA from each parent survived intact in the fertilised egg. The result would be as for model 1.1.1.

Model 1.2.2: Which is radically different from the "earth" system
The specific chemicals used in DNA are not important to the final product; the important thing is that the system recognises the chemicals as having meaning. An alien race could use double-stranded RNA, or perhaps something more esoteric: a crystalline structure, or protein based "chromosomes". the complete lack of similarity here would again render crossbreeding practically impossible.

Model 2: Ebons are a breed of H. sapiens
If ebons are simply humans from a different gene pool, then cross fertilisation is not problem. The question is whether, and in what form, ebb abilities will be transmitted to the offspring.

Model 2.1: Ebb abilities are polygenic, continous traits
In this scenario, the full ebon genotype contains many genes which contribute idenpendently to an over-all ebb sensitivity. It would be logical, therefore, to assume that a crossbreed would contain around half of these genes and so have "half" the ebb ability of its ebon parent.

Model 2.2: Ebb abilities are polygenic, forming a single "ebb" faculty
If the ebb abilities of an ebon result from the formation of a single node of the brain, or even a single protein in the cells, then the contirbutory genes would all need to be present intact; a single flaw could ruin the entire process, or at least render it severely disabled.

Model 2.3: Ebb abilities result from a single gene
If there is one "ebb" gene, then according to the laws of Mendelian heredity, an offspring will have a 50% chance of inheriting it. It will be passed on intact and in full, or not at all.

Other Questions:
Other racial characteristics of Ebons
The glowing eyes, pale skin and small frame of most ebons would seem to be recessive traits in a human gene pool, yet they are nearly omnipresent in ebons.

Karma
A further area for consideration would be the ability of the ebb itself to transcend genetic limitations.