Depending on region, the Portuguese have 5-10% sub-Saharan maternal lineages (L haplogroups). M1 is East African in origin, so the Central Portuguese have about 11% Negroid maternal ancestry. (Pereira et al. 2000)
In addition to Negroid maternal ancestry, Carvalho-Silva et al. (2001) found sub-Saharan Y-chromosome haplogroup 8 in Portugal at a frequency of 1.1% (n=93).
Other studies have failed to detect sub-Saharan Y-chromosomes in Portugal. However, I think it may be significant that the study cited above did detect HG8 in a sample of Portuguese. Apart from Rosser et al. (2000), which found HG8 in Sardinian and French samples, and another study which found sub-Saharan haplotypes in Corsica, I believe this is the only time I've seen a sub-Saharan Y-chromosome detected in a European population (Update: Greece is now on the list of European countries in which sub-Saharan male ancestry has been detected).
In any event, the Negroid paternal contribution in the Portuguese is much smaller than the Negroid maternal contribution, which is to be expected if the Negroid strain was introduced into Portugal as a result of the Atlantic slave trade.
The Negroid genetic contribution in Portugal could be something like 3-6% of total genes. Though an extraordinarily high level of sub-Saharan ancestry by European standards, this ancestry is minimal enough that it seems unlikely that it markedly affects the phenotypes of most Portuguese. But, through recombination, it's not inconceivable that some Portuguese exhibit Negroid traits due to slave admixture.
Also, the fact that levels of Negroid ancestry in Portugal apparently vary by region tells us that the distribution of Negroid genes is not uniform. If there is also genetic structure by sub-region and social class (as seems likely), there is a reasonable likelihood that some groups of Portuguese are greatly above the national average in their levels of black genes, and those groups might show external signs of their Negroid ancestry. Update: This prediction of mine seems to have been vindicated by a recent report that "sub-Saharan input" is "above 20%" in certain rural areas of Portugal, "where black African physical features occasionally mildly manifest themselves in the natives". I don't have a full citation for this report, which was apparently transcribed from a print journal and posted to the web by someone intent on minimizing the importance of the sub-Saharan component in the Portuguese, but no one disagrees that these native Portuguese with manifestly sub-Saharan features exist.
Update: According to the latest paper on the subject (Gonzalez et al. 2003), some of Portugal's Negroid admixture may date to the Neolithic. Guenther (see below) was probably correct to ascribe the Negroid element in Portugal to both ancient and recent sources.